There is a great deal we still do not know with regards to the exact signals used by the body regarding the genetics that tell growth of various body parts to stop growing during puberty.
If for example we get a "stop" signal reversing this might be quite difficult, if we instead were to simply tire out receptors and get what would amount to a "lack" of a "go" signal then overcoming this may be something within our grasp within the next few years.
Pubertal growth is made up of a combination of processes. You have hormonal stimulation of cellular growth combined with stretching caused by frequent erections which are far harder than what an adult needs for sexual intercourse. What these do is to stretch out the corpus cavernosum and ligament structures when they are still pliable and able to be stretched. If we were able to soften the ligament structures (some say they can be broken down using para amino benzoic acid) maybe additional length could be obtained. To accomplish this in an adult we would now have to have several things that would have to take place to make it possible. 1.) We would need to be able to activate dormant hormone receptor sites signaling them to go into a growth mode. 2.) We would have to be able to soften the ligament structures to where they would return to a more pliable state as they were when in puberty. 3.) We would have to recreate the degree of erections necessary to stretch the corpus and ligaments from the inside out. This would renew growth of both length and girth growth.
#1 is not completely understood yet though there might be a few possibilities. #2 has some possibilities, but additional studies would be needed on the long term effects of heavy dose transdermal applications of para amino benzoic acid with regards to safety.
#3 can be done easily and we have the technology to do this part easily right now.
Those working on what is referred to as "Chemical PE" these days are working heavily with #'s 2 and 3, but they have not gotten #1 completely licked. There are some claiming success, but safety is questionable and not really documented as being either consistently safe or consistently effective as yet. There is some evidence however that this might be at least somewhat effective. Time will tell. . . .