Hm, i'm not sure. I'm a young sub. Maybe when you're in your late teens/early twenties you feel a bit more sexually liberated? By your mid twenties you might be at the stage where you've found the right dom and settle down? Far be it for me to cast aspersions, but possibly you might be slightly less likely to enter the bdsm scene in your thirties.

This is a vanilla problem too, lots of men find that by the time they want to settle down/ start a serious relationship "all the good ones have gone".

It's a good question, i imagine that bdsm demographics would be interesting to look at. Just my thoughts anyway.