let's make one thing clear: all porn you can find is legal, i.e. >18. they are paid actresses. no producer will risk going to jail for that. not in the western world at least, where that law is enforced.
2. normal people can't find real underage porn. I wouldn't know where to start looking for it, but those that are really interested will find it if they want it badly enough
3. porn, just like everything else, is marketing. things come and go. 20 years ago, the word 'milf' didn't even exist. now the 'milf' thing is played out. how long can men watch milf movies before they get bored? so the producers will move to something new that gets some traction. that snowballs into being the norm. then it's played out, and something else comes through, or comes back. look at shoes, dresses, mainstream movies. everything is a cycle.
4. there are lots of millennials that are avid porn consumers. all the statistics say they're not having sex, so I bet they are jerking it hard. they are not interested in looking at 40 year olds, even if they are in shape. they want to see fresh meat, something that they would find in any given club.
5. never underestimate the power of fantasy. some men imagine they're 15, and bang that 14 year old, when in fact when they were 15 they were pimple faced and blew their load if a girl said hi. sure, he's 50, and the girl is 25 in the vid, but who's gonna know?