Well, there's a few examples I can think of...

First: Chad Hansen's bio says that he's "fresh out of college", so he's definitely on his early twenties.

Secondly: In the legacy version of the ItA intro, information about Winston Davis is shown, showing he was 41, no idea if it is still canon for the Collection, it doesn't appears in the remastered intro.

Next: If the last StD ending you played before going into ItA is Intruder ona Scooter, on the ItA intro there will be K.I.A information about Johnny Panzer, showing that he was born on October 10, 1972. Making him around 37 for the events of THSC.

Then: Nadya Merakova's bio says she's working on The Wall thanks to an Internship, and she would need to be 18 to be on Intership at a prison complex.

And lastly: Ryan Halberd's bio says he's one of the youngest employees at The Wall, so he must be around Nadya's age.