It depends, again, what to mean by sex. The author of the question meant only the penetration of the vagina by the penis, or, after all, we will consider this concept somewhat more broadly? And then, you know, if you imagine the sex of a miniature nymphet aca loli, at the same time quite an adult, and a two-meter macho with eight centimeters in diameter, then here it is clear what will happen ... and this is no longer a question of age, but of dimensions in general. If the dimensions do not match, everything here varies from mild discomfort to injuries incompatible with life (I know of one such case, in fact there are many more).
From a psychological point of view, as my colleagues indicated above, everything much more difficult. To begin with, any bodily contact is, in one way or another, an invasion of personal space. A person who is developing normally psychosocially will regard the intrusion of a stranger into his personal space as moderately negative, in extreme cases neutral - this is a normal, healthy defensive reaction. Even overly obsessive caresses from close people can cause rejection - because even if the consciousness does not understand what's what, the subconscious mind gives a signal that there is something wrong here.
And now, proceeding from from the above, directly about sex: in order for the matter to be without trauma - in this case, I mean psychological - it is necessary that a person has formed a mechanism of sexual attraction, which is a kind of "pass" for a close other into his personal space. It is difficult to name the exact age when this mechanism is formed, but the figure is usually called 14-16 years. Only for some it happens later, for some it happens earlier.
If everything is okay with this, if a suitable partner is found, then there is only a question of approach. The brain may already be ready, but the body is not quite yet, plus the inexperience of partners - in general, it all comes down to the fact that sex in general comes down to the aforementioned penetration. In my opinion, this is the reason for the poor attitude to sex in a large number of women - the first experience is still stored in the memory stronger than the previous ones. And in general, I would not recommend starting the first sexual contact directly from penetration - and ending with it. By the way, no one has canceled the prelude, every petting there. And this, I believe, is not child's play, but a direct necessity, because, firstly, it allows you to evaluate the partner's body, and secondly, it gives you the opportunity to relax, which means, indirectly, increases the level of trust, and thirdly, it makes it possible to stop , if suddenly some lever clicks in the head, reporting discomfort. With this approach - but here this is my personal opinion - there should not be any mental trauma.
In general, there should be some kind of benefits for adolescents at the age when they, on average, begin to be interested in sexuality. And these guides should provide more detailed advice than those recommending condom use.
From the point of view of physiology, sex before puberty is harmful, because it is more traumatic. The readiness of the genital tract for sex is fully revealed in the process of puberty.
The psychological aspects of sexual relations are highly dependent on what patterns are common in society. For the psychological norm is in general in many ways a matter of social acceptability.
Unfortunately, a question that implies an answer "yes" or "no" - they are also called "closed" - does not imply any facts in its answer)) Only personal consent or disagreement of the respondent.
I immediately bring apologies for the impression my words may make - believe me, this is not cynicism, I am just trying to be objective.
From a physiological point of view, the harm from sex occurs when an effort is applied to the body of the participant that exceeds strength its tissues. Those. if the dimensions are too different, there is a high probability of causing physical harm. This is not due to age, but precisely to the size of the body and corresponding organs.
Of course, with age, the size of a person increases (up to certain limits), so the likelihood of physical injury decreases with age.
With psychological herbs, everything is much more complicated. Now I will say an unpopular thing, but psychological trauma occurs when events happen to a person or actions are performed with him that are contrary to what he considers right and good. If a person does not know or does not believe that something bad is being done to him, psychological trauma does not occur in principle.
Nevertheless, sex has been associated with violence, domination, power, obedience, and monotheistic religions over the past couple of thousand years have added to it such epithets as "sinful", "shameful", "shameful".
At the same time, children are traditionally brought up in a sex-negative way even today, at best, simply taking him out of the context of communication with a child, at worst, they openly taboo the topic of sex for a child and punish and shame him for showing even simple curiosity.
This is supplemented by the ubiquitous sex-negative abusive vocabulary that lays down negative attitude towards sex and genitals on a subconscious level, long before a person realizes what these words generally mean.
However, with age, people tend to get rid of dependence on the opinions of others, more crit rationally assess the surrounding reality and rely less in their judgments on their own experience and thoughts, and not on other people's patterns of behavior and reactions embedded in the subconscious since childhood. Therefore, with age, negative attitudes towards sex become less, reducing the likelihood of mental trauma.