Can I ask why you check out women in 20s - simply because they're attractive? What about women in their 30s? How old is your DP?
Sure, no worries. My DP is 32 - one year older than me. We met online because I did the usual "messaging someone who had something at least a little bit interesting to say and met my minimum physical standards" thing and we got chatting very quickly, often for hours at a time! We're LDR now because I work outside the country 9 months per year, but have been together for a year and a half and all is going well! She wants me to wife her (her phrasing haha) and I'm inclined to do so, assuming we can come up with a plan for the future.
When it comes to looking at other people, I can only speak for myself, I don't claim to be representative of my whole gender... However in my mind there's a very clear dividing line between appreciating someone's physical body and looking at someone and thinking "potential partner". I appreciate people's bodies all the time - street, work, beach, everywhere. In the same way I appreciate anything that's aesthetically pleasing when I see it - art, motorbikes, nature etc. However I very rarely think "potential partner" without having spent a decent amount of time getting to know someone. However when "just looking" occurs there's really no particular age limit that's strictly defined. I can appreciate a pretty 18yr old just as much as I appreciate a pretty 40yr old, but it's not a lust thing, it's just... liking the curves, I guess? I wouldn't go anywhere near someone at the extreme ends of that spectrum in terms of relationships, and it would take a conversation, and the possibility of further conversations in the future, for someone to transition in my mind from one to the other.
That's why I suggest clubs and sport and societies, because it allows you to talk to guys and make that transition, rather than just go places where everything is 100% based on looks without any conversation taking place first. The prospect of seeing someone again in the future instead of them vanishing is another important step for me to think of a woman in terms of relationship option rather than "pretty but not possible".