An interesting series of studies suggested that various issues with aging were actually due to lowered muscle mass, rather than issues in aging affecting muscle mass.
However, testosterone was not one of these factors, being that T production resultant from weight training tends to be lower in older age groups than young men.
With TRT, however, this, too, should not be an issue.
In terms of muscular gains, if all else is equal (Therapeutic use of HGH, T, t3, etc.), then I believe one's muscular peak is more dependent on injury accumulation and susceptibility due to training age than age alone.