Well ideally you will train for a long time and master your training for your genetic type and master diet before you use steroids. I think that is more important than age... with steroids the risks concern people who aren't physically developed or fully grown mostly... 21 is an arbitrary age made up by supplement companies trying to cover their asses from lawsuits and doesn't mean anything scientifically.
There are 18 year olds who are physically and mentally ready for steroids (although not very many!!) and there are 25 year olds who are neither physically nor mentally ready for steroids.
Personally i started doing PH's at 20 and i'm 23 and i've done like 8 cycles in that time period and I think I am no worse off from it, i am not prone to most side effects and by the end of my PCT my test levels usually feel like they exceed pre-cycle levels. That said, i knew exactly WTF I was doing, I was obsessed with understanding everything, my body matured long ago and I was at a high level of training... And even so I know that I "jumped the gun" and that I'm impatient, a fault I cannot help.
Most people that are 20 aren't ready, they are mentally immature, do not have a good diet, do not train consistently (consistently meaning more than training when on cycle... meaning training every week for YEARS like me), and do not push themselves enough to make the best use of a cycle. They don't understand how steroids work or the side effects or how to combat the side effects, or just HOW a PCT works and what exactly every component is doing. They are just doing what their friends told them! Like you, you can tell your friend "yeah take havoc, take this for support, take this for PCT" and 95% of the time it will lead to a safe cycle with good gains and no problems, but sometimes people will get gyno that won't go away, or they finish up their PCT and realize that they've lost their gains, they're depressed, and they can't get it up! You don't want to be responsible if that happens to your friends.
I encourage anyone who wants to cycle to research everything for themselves. You shouldn't just cycle because your buddy says "take this and that, it'll be safe." You should understand WHY you are taking X, Y, and Z, what it all does. You should take full responsibility for yourself and understand what you're putting into your body.
But most importantly, only cycle when your training and diet are GOOD (and most people who cycle still don't do their diet good enough IMO) and you have pushed close to your genetic limits. Like I said most people aren't ready when they are 20.
But I believe in the dream of America and being free and if they wanna pop havoc then so be it... Personally I think 20mg is a weak dose and will probably leave them disappointed, IMO if you are going to do something, do it proper, start at 20mg go to 30mg, run a real PCT with a SERM. Just tell your friends to be responsible for themselves and research what they're doing, not just to take your word for it because you asked the Bros on some online forum how to run it safe.
Nothing is 100% safe here! There are no guarantees, everyone takes a risk whether you are 18 or 40 and a noob or experienced... Be smart and responsible.