Guts when he joined the Hawks - 15
Guts when he left - 18
Guts when he came back / Eclipse - 19
Two years pass from the time of the Eclipse to when we see him in the first Volume. He's 21 at this time. From Vol 4 - 13 it takes place during his early years and time when he was with the Hawks. Vol 14 picks up right where Vol. 3 ended (with him defeating the Count and leaving with Puck). It's even stated that he's been gone for 2 years from the time of the Eclipse to when he returns back to Godo's mine. The rest of the manga takes place within a year or two's year span (giving time for him to travel all around Midland). So at most he is 23. Otherwise characters like Rickert, and Erica would have aged quite more than they did.
tl;dr Gut's not in his early to mid 30's.