IMHO only when you hit your natural peak or if you have been working out a serious 4-5 years.., 20 years old to start if you really think you have to use, again imo.
I would wait until you are in your late twenties when your own bodies production of natural test is on a down fall. Even with that I would be very skeptical or using prohormones. I have tried most of them including the cyclo-andro poppers by Pinnacle and the Androsol by Biotest. All i got was a little energy and a little strength. With the side affects I dont think it is worth. As far as the 1-ad I cant speak of that because I used it once and I thought my stomach had a hole in it. Shit hurt big time and that was it for me. I hear some say it is great though.
Age alone is not enough for me to make a good assessment. Some 18 year olds are late bloomers and still have some adolescent growth left into their early twenties. Some 18 year olds OTOH have not grown an inch in height, and have been shaving, for ten years already.
If this guy feels he has pretty much physically and EMOTIONALLY matured at his age of 18 then he can consider prohormones.