You're dead-on on the age thing; everyone at the gym is in their 20s and early 30s, except for her boss, who is late 60s (and in phenomenal shape himself). So it's not like she's chosen this group of younger people, it's that EVERYONE there is younger.
But she IS definitely hung up on all things "age" related. We have a nice steak restaurant chain here, Stonewood Tavern & Grill, that we all like and even my D20 picked it as the place she wanted to have her 18th b-day dinner, but now my wife doesn't like to go there because "there are too many old people there." I'm like, WTF? what difference does that make?
Another wrinkle to this is that the guy who was turning 21 is a cop, doesn't actually work at the gym but is best friend's with my wife's trainer, and he is recently now dating my D18 (who obviously couldn't go last nite, but was invited, and she said "Oh great; what am I going to do, DRINK CHOCOLATE MILK??").
-- Choc., who wants to know what's wrong with chocolate milk?