The GMAT is offered year round in many centers around the world. You probably only need a few weeks lead time to register to secure the date you desire.

Some schools do limit the timeframe to 2 or 3 years, but most will accept any valid scores (so 5 years). If you already have an idea of the schools to which you'd like to apply, visit their web sites to check their policies (though there's no guarantee they won't change policies at some future time, of course).

It's a really good idea to get the test done sooner rather than later for two reasons:
1) On average, people do better on standardized tests the closer they are to their college years. For every additional year out of college, average scores drop.

2) The schools are actually starting something of a push to get younger students into b-school, back to 2-3 years of experience on average (as it used to be) instead of 4-5. I was at a conference in October at which the president of GMAC spoke (GMAC is actually a coalition of business schools - they started the non-profit GMAC which owns the GMAT). He specifically said that they want to attract younger students and are putting programs in place to make this happen - you may have heard that Harvard recently started a program in which you can apply to HBS your senior year of college. If you gain admission, you get an automatic 2-year deferral and they can help to find you a 2-year job until you start b-school. They even wave the application fee for college seniors. Check it out!

Nope, I can't, unfortunately - we don't do admissions stuff, just test prep, so it's not something I keep up on. I don't even know if any still do this today; I just know there have been instances in the past. Again, if there are any specific schools about which you're worried, check their web sites or email the admissions department (but only do that if you've checked the web site first and the data isn't posted - never ask an admissions department a question when the answer to that question is openly posted on the web site!).