Joel, it is really good that you are taking the time to research your options well ahead of time. There are some incredibly knowledgable men on this forum. I am not one of them, lol, however, I have some personal experience with using PH's at a young age (20)... A year and a half later I still am making solid gains in the gym, I am still a fan of PH's due to the fact that my current legal situation makes it dangerous for me to be involved with AAS.
I will do my best to answer your specific questions
1. You are right, this completely varies from person to person. Obviously diet and training are the biggest factor in you meeting and keeping your goals. Take this time to tweak your diet accordingly.
IMO, the only PH's that are worth a shit... well the only ones that have proved worth it to me, are methylated orals. They are hepatoxic. There are preventative measures that can be employed in order to try to neutralize the risk but most people still experience in increase in liver enzyme activity. Bloodwork is a good, relatively cheap way for you to understand the effect of the substances on your body. There are some non-methylated PH's. Two that come to mind are the "19-nors"- products that the PH companies like naming "Tren"... i'll get to those in a minute. There is also Bold 200, which I have heard some great things about. The only downside to bold that I am aware of it is apparently pretty underdosed, making an ideal cycle rather expensive.
"Tren"... First of all, the name pisses a lot of people off as it is all of these companies attempting to capatalize off of the reputation of Tren Acetate, a form of AAS. I have more personal issues with these substances. They appealed to me at one point because of the lack of hepatoxicity. I did a 8 week cycle of trenaplex 75 mg a day. Gains were decent. I got some good vascularity, muscle hardness, and some decent strength gains. The problem came after PCT when I developed a slight case of gyno in my left nipple. Speaking with other people, this seems to be a pretty common issue with this substance. I will forgo reccomending you a specific product as I really don't know what your goals are, and if I were you, and AAS was readily available, I would use it for my first cycle.
The endocrine system is such a complex area of the human body that there aren't a lot of set in stone rules. Obviously, no one, besides a doctor could make the judgment as to whether or not your endocrine system had mature from puberty and entered a level of homeostasis. I did PH's at 20 (4 months before 21)... 4 months after my PCT my testosterone levels were higher than when I was 19, and having some symptoms of low testosterone. Every person is different. I don't know who established the age of 21 as a relatively safe one to begin playing with your body's hormonal makeup. Some people would argue that it is never safe to introduce additional hormones to your body.
As with all elements of personal fitness, educated decisions, as you put it, are the key to safely, and efficiently reaching your goals/full potential.