Its kind of strange while Franklin and Micheal's ages are indicated, (25 and 45 respectively) nobody has really said anything about Trevor's age. I personally think because he's so insane his backstory doesn't even matter. Nonetheless, I have seen nothing about Trevor's age on any Wiki or part in the game so far.
You may think "Who has time for that sort of detail?" I do, clearly. There are GTA completionists who desire to collect every last possible detail about the GTA lore. Just think back to the LCPD database in GTA IV if you have no idea.
Anyway, for those into this detail, how old do you think Trevor is. I personally think he would be in his late 30s to early 40s. He was clearly Micheal's best friend since before he met Amanda (Remember the story)
EDIT: Mike was said to be the oldest GTA protagonist to date so Trevor can't be any older than 45 (lel)