No, he wasn't in his early 30's. He was 39 when the Towers fell. Being the Mac nerd that I am, I put together a timeline for him. While we've never been explicitly given his age in the series, it's fairly easy to guess based off a couple clues. He mentions that he was a new Lt. when he was in Beruit. Given that and his referenced college science background, it's fairly safe to assume that he went to Marine ROTC and commissioned in as a 2nd Lt. Making him 21 y/o in 1983 (Beruit). From here the rest is fairly easy.
1962 – DOB
1983 – Marine 2nd Lt. (21 y/o)
1987 – met Claire (25 y/o) - just a guess, but a pretty safe one I think
1990 – married Claire (28 y/o) - given his newly-wed comment to his father in 'Blacklist'
1991 – left Marines, took NYPD job as beat cop, father died (29 y/o)
1994 – made detective and joined crime lab (32 y/o) - best guess based on references in s7 and his lightning speed up through the department
1998 – made detective 1st grade, Stella joined lab (36 y/o) - the 1st grade is again a best guess, but Stella joining the lab date is not
2001 – Claire died (39 y/o)
2002 – became head of crime lab (40 y/o) - again, best guess on the year based on the fact that he seemed fairly new yet not brand new to the position in s1, and that he couldn't possibly have gotten to this position any sooner given he was only out of academy 11 years! It wasn't even as if he had prior police experience coming into the department...!
2003 – ‘Blink’ (41y/o)
And given that he was only 29 when he and Claire got married, I don't see her being hugely younger than he was. Younger for sure, but I really don't think it was more than 7-8 years. She was a college student where he was stationed, or something like that.
And on a spoiler note, I was at the Lt. Dan Band concert in OshKosh, WI last night, and Gary mentioned out they just finished shooting 8.01, and said that they've included the Wall of Remembrance Memorial (where the pictures and plaques of all the first responders who died at 9/11 are) as a location in the episode. He said he'd been bugging the writers/staff for two years to get them to include it!