The question is not entirely correct. But I will answer in order.
Somatic (bodily) harm. And an old man can get a heart attack from a cosmic orgasm, and a heart attack is really harmful. Omitting such ... exotic cases, in the orgasm itself there is nothing harmful at any age. Another thing is that vaginal / anal / oral / any other type of sex for the receiving party can end in injuries, starting from ruptures (anal fissure and so on) - here it is more likely from experience, the emotional state of partners and other things that depends on age. STDs are from the same series. Here you can cite the connection that more ... young people may be more dismissive of these consequences. Although, what can I say, and "adults" often put coconut on such seemingly obvious things. So in this regard, I would say that it is not harmful from any age if TB is observed. Since the age when the reproductive system of the body has developed naturally. And usually this age is much less than those 16-18. Conventionally, we can say that it is somewhere in the area when a boy / girl can and wants to masturbate. And what's wrong with masturbation itself?
From a mental point of view. The main thing is that a person himself wants this and be responsible - and not everyone, even at forty, follows these simple rules. Therefore, here the link to age is questionable. As, for example, psychological trauma (violence, harassment) or promiscuity (promiscuous sex life).
Remember the ancient culture, where young boys were considered quite mature for sexual intercourse. Therefore, the main part is played by the opinion of society. And now in some countries sex is legalized from the age of 14. And for me, imprisoning another American teacher for having sex with guys is absurd. I do not see anything so terrible in the same ephebophilia . But this is not the same as pedophilia. The second - there the object of attraction is not young men, but children. Even physiology suggests that children (in this sense, who have not yet developed the reproductive system) are clearly not an object for sex. And this pathology is one of the most terrible, in my opinion.
In short, it is not harmful if a person is ripe for this morally and physiologically, and this is done by mutual (triple, quadruple, etc.) consent. Without reference to a specific culture. Just please take care of your protection and do not forget that the criminal code applies regardless of your opinion. It is very harmful to sit in jail.
In puberty ..... for health it will definitely not be harmful, for the physical. But in psychology - it depends on the person, each in his own way. Someone leaves innocence already at 13 and does not regret anything, and someone is afraid to do it even after 18, and having done it, they become depressed for various reasons.