The actual natural physical prime of men considering healing ability, explosiveness, strength of the heart and nervous system, etc. is between the ages of 16 and 24. In most men the frame - that is bone structure, attachments, etc. are fully grown and mature by the age of 16 roughly, and testosterone is naturally at it's highest and nervous system strongest until the mid twenties, hence 24.
Alot of people don't like to accept this. But it's reality. PEDs can extend this prime significantly though.