Only one of my 15 or so suits is three piece (only one is double breasted as well). I'm usually too warm and never too cold when wearing a suit. My three piece is a HF in a winter-weight gray hound's tooth. I only wear it during the winter months and get a lot of complements when I do.
i'd agree with most everything said here - it's not so much a matter of age, it's a matter of opportunity and use. i know i rarely wear my suits, and so the idea of buying a three-peice (although i love the look) just isn't a good use of my money. i have a tendency to completely disagree with GBR - i think the three-piece is definitely fashionable/in style right now, independent of age.