If you're asking at what age guys usually look their best--it would be between 35-40. But, if you're asking at what age the physique has the "potential" to look its best, then it would be the mid-later 20's...no doubt.
However, the only reason most guys end up looking better in their 30's is because BB'rs usually can't gain enough muscle mass before that time. But, if muscle mass was at its peak, then the 20's is easily superior.
By the time a BB'r hits his 30's, he doesn't look as fresh as he used to and the waist doesn't look as tight anymore. The entire body loses some of its "prettiness". In other words, one's natural aesthetics begin to deteriorate. Seperation starts to blur, especially in the legs. All in all, basically everything, aside from muscle "maturity" (and obviously muscle mass), goes downhill.