Wow this is a real generalisation and while true for some, it will likely be the opposite for others. For example, I know guys who in HS were precocious physically so tended to look older and therefore attract girls in our year or younger but nowadays couldn't pull a granny. However, there were guys who were that little bit more quiet, alternative, mostly not into sports and thus weren't seen to be as popular but once they went off to uni and escaped the bubble of HS, things became easier.
Now for guys moving into their 30's then this can go one of two ways broadly speaking. Either you're looking pretty burned out and old for your years which is quite possible if you were already growing a beard in HS or you are a slow burner who tends to come into your peak in your 30's in terms of physique and looks. Of course, if you have a decent job, life experience and a nice personality then things will be all the better for you.
Then there will always be guys who are just naturally successful with females for a whole variety of reasons - insanely good looks, great sense of humour or just something unique about them that draws a lady in - these guys are < 0.1% fortunately for the rest of us.
Personally, I've never had a major issue attracting females at any age and dated two of the hottest girls in my final year of HS and even turned down another hottie as she was best friends with an ex. I wasn't precocious but was pretty smart and hung about with a decent bunch of guys who treated girls with respect.
University was even better and it was pretty easy to get laid especially with our friend booze in your company.
However, for me, I seem to now becoming into my peak years at 31. Perhaps because I look pretty good for my age but possibly because I just don't put too much pressure or emphasis on it: go out with friends, have fun and if you get chatting then make the most of it.
It's the age old question really - how to pull women. No one guy has all the answers but for sure there are some pretty solid ground rules and it doesn't take much to get ahead of the field as most guys are pretty lousy.
Dress well, keep well groomed, keep in shape, don't take yourself too seriously (self deprication is a great tactic!), find common ground, treat the lady like an actual human being and never ever give the impression that you are desperate. Oh and for sure, get up and dance as no one likes that creepy guy standing at the back staring while furiously downing jaegers before hitting the floor like an elephant.
Happy hunting!