Ok, I did a fair amount t of RL fighting up into my mid 30's but not active now, so maybe shouldn't be offering a view. However:
It is both that the women who do this are and has always been a tiny fraction of 1% of the women under 40 in the USA, and also changing social climate has increased the opportunities greatly. Clarressa Shields won gold at the 2012 Olympics, the very first time women could box as an Olympic event; but the sport, along with some MA is now there in the Olympics. Also women even earning a pro living in MMA fights at the highest levels in greatly increasing: Gina Carano, Rhonda Rousey, Miesha Tate and many more. But this is, of course, not the bite n scratch type of fight most wanted here and, it seems, by guys in general.
Yes, this site is a self-selected concentration of people interested in women fighting,and not at all a reliable indication of actual women who actually fight. Age imposes its limits, but not as much a matter of under 30 as you seem to think.