I've found this thread very helpful in finding videos and identifying which porn stars I want to use for my own game. Girls who could pass for college age are especially difficult to find content for from what I've noticed because for a real porn game you need a wide variety of different types of content to have enough options to pigeonhole her into your story. But most girls have a very limited window of time where they could pass as college age. So I found that when I started looking through the videos I had, I had a TON of options for MILFs I could use that had lots of content but most of the girls who could pass for coeds has a MUCH smaller selection of scenes.
From what I've noticed, girls with smaller frames tend to be able to pass for younger longer. Girls with a build like Jenna Haze, Foxy Di, etc. By contrast the more voluptuous girls tend to quickly reach a point where they really are only believable in a MILF role.
I find myself having to put a lot of the younger girls into very minor roles where they're just a 1 night stand you only see once, or the slutty friend from out of town who only has 2 or 3 scenes. They pretty much have to be sluts or you can't use them because there's not enough content for them for you to build up to anything. That or you have 3 videos where they look 20 and 20 videos where they look 40. Just doesn't work.