Figure it out by 30

A piece of advice that I received during my first "real" job, at Coopers & Lybrand, was that I had until 30 to make political gaffes that could be chalked up to immaturity.  After 30, people expected you to know better.

Now that I've passed that point, I think that's it's a good general rule to follow.  More good reading on this idea in a post from the Brazen Careerist (focuses mostly on what it means from a female point of view).