The Protagonist is posibbly 31 or 32 in 2012(at most) because
- it was confirmed that he was in high school in 1996 and
- the first game takes place in 2003 and
- he also said that he went to college wich makes a 7 year gap
- which starts in 15 in 1996 because of the protagonist clamed that he went to highscool at that time and
- between 2003(time of the first saints row game) making 22 subract 7 equals 15 in 1996 at most and
- he did say that he was a kid in a trailer of Saints Row mybe passing college that year at most.
Compelling argument eh, so that makes makes the protagonist at most in 1996 15 yrs old making him 31 or 32 in 2012. 31 is arguably the most possible.
(Janitor's note: I've split this block of text into points, for readability)