No. Anyone of any age may edit articles or register. Wikipedia does not even require that users disclose their age when registering!
Note that users who identify themselves as minors (generally those under 16 years of age) are encouraged, and will be required, to protect their identities via safe practices where the posting of personal information is concerned. See Wikipedia:Protecting children's privacy.
What is the difference between a page and an article?
The term "page" encompasses all the material on Wikipedia, including encyclopedia topics, talk pages, documentation, and special pages such as Recent Changes. "Article" is a narrower term referring to a page containing an encyclopedia entry. Thus, all articles are pages, but not all pages are articles. See Wikipedia:What is an article for more.
An orphan is an article that no other article links to. Such articles can be found on Wikipedia, but it is preferable that another article should link to each article. You can find a list of orphans at Category:Orphaned articles.
When editing a page, a logged-in user has the option to flag an edit as "minor." You should flag an edit as minor only when you believe it is an edit that no one would object to, such as fixing spelling or grammar.
This feature is important because users can choose to hide minor edits in their view of the Recent Changes page, to keep the volume of edits down to a manageable level.
You could merge them yourself if you are feeling bold. Pick the most suitable page name (which may not necessarily be one of the existing ones!). If you're not sure which name to use, or whether the two articles should really be merged, use the procedure at Help:Merging. You can also make a mention of the problem on the list of Wikipedia:Proposed mergers.
What is the ideal/maximum length of an article? When should an article be split into smaller pieces?
On the English Wikipedia, use English, unless you're mentioning a name or abbreviation that has no known English translation. If you want to write using other languages there are many other Wikipedias in different languages. See Multilingual coordination for links to these versions. If your language is not active yet, and you would like to change that, read the language proposal policy to find out how to create a new language edition of Wikipedia.
The official policy is to use British spelling when writing about British topics, and American English for topics relating to the United States. General topics can use any one of the variants, but should generally strive to be consistent within an article. See Wikipedia's Manual of Style for a more detailed explanation.
Use of one English variation in article titles can cause a Search in another variant to fail. In this case, it is recommended that you create a new article using the alternative spelling that redirects to the main article.
A spelling checker has been requested for Wikipedia, but has not been implemented yet, except as a third-party extension. The Firefox 2.0 web browser automatically checks spelling in forms such as Wikipedia editing forms. When editing a larger article, it may be more convenient to paste the text into your text editor or word processor first, to edit and check the spelling there, and then paste your corrected text back into Wikipedia editor to complete your contribution. You can also use an online spelling checker.
There is a list of common misspellings, which you can use to check if a listed misspelling is on any page in the database, although this process fails to identify any misspelled words not on that list.
A page with that name has not yet been created. You can click on that link and start a page with that name. But be careful: there may already be articles on similar topics, or an article on the same topic under a different name. It's pretty important to hunt around for similar topics first. See Wikipedia:Article titles for information on naming pages, and Wikipedia:Red link for more guidance on red links.
If you just registered, your username is probably shown as linking to a page that doesn't exist. Don't worry! This just means you haven't filled out your user page yet. Click on the link and tell the world all about yourself! See Wikipedia:User page for more information.
This is called an edit conflict. You'll get a conflict screen that displays both versions in separate windows, along with a summary highlighting the differences, and instructions on how you should proceed. It's virtually impossible to lose any data.
What happens if my computer or browser crashes mid-edit, or if the server does not respond?
You'll lose your edit. To some extent, you can guard against this by editing in a text editor, for major work (but note that with regard to a system crash this does not help, unless you save frequently to disk).
When you get a time-out when you try to save, you might or might not lose your edit depending on your web browser. Some browsers (e.g. Opera and Mozilla Firefox) will recover the text you have tried to save if you use the back-button. In other browsers you will lose your edit. You can protect against this by copying the text (at least to the clipboard of your system).
How do I learn about changes to certain topics without having to go there from time to time?
If you are a logged-in user, on every page you will see either a link that says "Watch this article", or a small five-pointed star next to "View history". If you click on it, the article will be added to your personal watchlist. Your watchlist will show you the latest changes on your watched articles.
What file formats should I use for pictures/videos?
Per Wikipedia:Be bold, there's no need to wait at all. Simply make your change. If someone else disagrees with it, they can always revert the change, and then you can talk the issue out with that person.
Can I change the default number of contributions displayed in the "My contributions" list?
There are a variety of reasons (some common reasons). The first thing you should do is look at the history page for the article you edited. This will tell you who changed it, when they changed it, and hopefully a short reason why they changed it. If it says something like see talk, then you should look at the talk page for the article. Also, you should look at your own talk page to see if you have a message there. If you don't find a reason that is satisfactory, politely ask in the article's talk page about your proposed change, and maybe you will get suggestions about changes that you can make so that your change will go in, or you may get reasons why your change should not happen.
Links: external and multilingual
Should I translate pages across the various Wikipedias?
Yes, it's a good idea to cross-pollinate. Please give credit to the contributors of the original article by noting that you have done so, with a link to the original, in the edit summary or at the article's talk page. See Wikipedia:Translation for further information.
Machine translation is useful for obtaining the general idea of a text in an unfamiliar language, but it produces poor translations and should not be used on its own. If you want to use machine translation as a translation aid and intend to edit the result, please go ahead if you think it would be helpful. Do not paste a machine translation directly into an article.
How can I tell if an article exists in another language Wikipedia?
We try to build links between different language pages – that's one way of seeing if an article exists elsewhere. If you don't see the language links at the left of a page, go looking for the corresponding article(s) on foreign Wikipedias. If you find them, make a link both ways; if not, you can translate. Bear in mind that article may not be in one-to-one correspondence between Wikipedias. See Wikipedia:Interlanguage links and Wikipedia:Multilingual coordination for more information.
External links are certainly allowed. Properly used, they increase the usability of Wikipedia. Keep in mind, however, that Wikipedia is not a web directory; external links should support the content of the article, not replace it. An article should be more than a container for external links, and the content should not require the reader to leave the site to understand the subject.
Do not place advertising links in Wikipedia. Commercial sites are obvious, but this prohibition usually includes links to fansites and discussion forums as well unless the site is a notable one in the field. As a general rule of thumb: if you wish to place the link in Wikipedia in order to drive traffic to a site, it probably doesn't belong here.
The current convention is to place external links in a separate "External links" section at the bottom of the article. Sites used as references for the article should be listed under a "References" section, or sometimes placed within the article as a footnote. See Wikipedia:How to edit a page for different ways to create external links.
Yes, you can on almost any page. There are some pages on Wikipedia that are protected, so that only administrators can modify them. This includes pages like the Main Page, which are permanently protected, or normal articles which are temporarily protected during the resolution of an edit war. Some pages are also semi-protected, so that anonymous and new users cannot edit them, to reduce vandalism. The vast majority of pages on Wikipedia are editable by anyone, at any time, and any changes made will be reflected instantly.