I can't see enough of his face to warrant a decent guess. His hands and arms could put him anywhere up to mid 30's, maybe even 40. What I can see of the face looks a bit of a baby face .... and with that hair style and baseball cap he could be a teenager.
Next time take the cap off and smile for the camera and we might be able to have a better guess :)
Looks like he needs a good meal to me, those skinny ribs are a right turn off. As for age looks to me like a fifteen year old trying to look like a man but failing.
Old trying to look young, more likely a babyfaced 35 than a well muscled 19, highly unlikely a younger bloke would have the dedication to do that much gym work