I know he’s partly joking, but I wonder if this also has something to do with a lot of liberals seeing women under 30 as children (but, strangely, only women, not men).
I think this tendency also has to do with the fact that you’re most often hearing this claptrap from the most educated, most out-of-touch bits of the population, who spent their prime years in college and grad school, living like a kid off the parental largesse and wealth until they were 32.
Meanwhile, out in the real world, most people who don’t have fancypants lives are working hard jobs from the age of 16, having real experiences, and are not the grown children that the “elites” in fact are. Bet that’s a lot of it.