Steven is portrayed as being quite young, but I can't place his exact biological age. What confuses me is that he seems much younger than Connie who is portrayed as his peer.
All I could find is this:
Steven's exact age has not yet been confirmed. It was confirmed by Matt Burnett that we might not find out for a while. 
With no clue as to if that is a stale quote, and not being able to watch all the episodes through, I can't tell if this has actually been revealed yet.
I know he can affect his apparent age, with his mental state1.
How old is Steven?