I started doing push ups/working out from 12-13 years of age. I stopped for a while. My peak was like 22 push ups. I got pretty good arms, it has the curvy shape and the muscle looking like a fresh round oval-shape and all that. I'm 18 now, and no matter what, I can only push up to 20 average...my arms get really tired and weak and I give out after like 20-23. I had friends when I was younger who could even pull 30-40 without a sweat, and they're kinda skinny (but lean). Is there a serious problems with my arms?