The Over/Under 25 obsession with age gaps is really weird

There's this weird push for age 25 to be the adult age for everything on social media and that people don't/can't mature significantly after age 25. 18/23 is heavily criticized for some reason but 25/45 will be considered okay by the people pushing this even though that is a more challenging age gap and would be more noticeable in public.

Is this based on that brain matures at 25 myth still being spread around? There wasn't such a weird cult around this particular birthday decades prior. People just minded their own business and accepted that age/maturity worked more on a spectrum than a hard rule of "age 25, you magically shift".

I just don't get it.