I agree with the other posters... to an extent. XD I do agree that it's definitely all about the individual. While you're shopping for horses I wouldn't have that be a defining search factor.
That said, I will never purchase another mare. Ever. I have met too many witchy/moody ones, and they're just not my cup of tea. Sure, my gelding can have his moments (all horses can) but overall I much prefer the stereotypical gelding to the stereotypical mare.
As far as age... another tricky one. I've dealt with young horses that were steady as could be, as well as older horses that still acted spooky and green. By now, my 21 year old is a gentleman who acts his age, and who still has plenty of good years left in him.
How much experience do you have, and what are you planning to do? (I didn't see it listed elsewhere in the thread, but I could have missed it) 10-20 is usually a good age range for less experienced owners. 15-20 if you're /very/ inexperienced and/or don't want to put up with a greenies sillies.