As written above about drug addicts, the purest truth, with their eyes closed, they can let themselves through their veins.
And you can inject ... medicines or vitamins. But it's better to practice with someone and with someone. This will make it easier and safer to start.
Well, not all drug addicts have a medical education. So yes, you can. And according to observations, absolutely at any age. The main thing in this business is practice.
Judging by the fact that drug addicts inject themselves without problems, a person can learn. The problem is that with intravenous administration of drugs, the likelihood of a strong allergic reaction is higher, and without medical education it is quite difficult to save a person, and even more so yourself.