In his 20's? He grew into a show as late/recent as 2003 when he was 39. He's also 52 now not 51.
What I'd like to know is are steroids less effective at 52 than they are at 39 because Kevin is pretty much "all drugs", so if there isn't much difference, one would presume that if he were willing to go to the excesses he did back in the day, that he could possibly attain that "all drugs" physique again.
But from what he has said in his q&as and his general unwillingness to make a return to stage over the years, I don't think he is juicing as hard as back in the day. At least, I think he was trying to get away with that before - but I get the feeling as the show comes closer and more people say he's behind, that he will indeed amp it up. It'll still be too late, though..