I think these young ages are all great times to build a good base and begin bodybuilding. But all of the top level competitors don't begin to look like monsters till there 40's. Hell Branch Warren is 36 and Jay Cutler is 38.
I just think with the current level of competition it takes that many extra years to pack on that much mass. It's kind of funny how the top level guys use to be so much younger than now.
IMO: I would say your at your peak for BB (w/AAS) 30-36 years of age. Metabolism slows down making it easier to pack on weight. Test levels are still naturally moderate. I have seen many BB's that started late in life late 20's early 30's and within a few years they are monsters. Just my own thoughts though.
I'd say that age 19-20+ is when your test is at the highest and most efficient time.