This isn't true. In reality you only pay attention to who highly sexually attractive men date while average and below average men do not ping your radar.
Most 50+ yo guys still on the market do not date women in their 20's and are incapable of attracting them even if they wanted to.
This is true to some extent but I don't necessarily think it's about being "established" in terms of career/money if that's what you mean. It's more than men in their 30's are confident due to having achievements + experience with women while men in their 20s (especially in their early 20's) are unsure of themselves.
With the nerdification of young men, I think older men who "get it" are becoming more attractive to young women. May-December relations will continue to grow more common IMO.