This part is true.
In Islaam, the absolute latest age at which a person becomes an adult is 15. You can become an adult before the age of 15, but there is no becoming an adult
after the age of 15. This is something in which Islaam differs greatly from Kaafir society. Kaafir society will view teenagers (even 19 year olds) as still "boys", whereas Islaam regards the person to be an adult, a man, from the age of 15 already, or, if he had become Baaligh before then, then from the age of Buloogh.
This is something which may seem trivial, and you may wonder why this would even get posted, but it actually holds a lot of importance and therefore I am writing this. The Kaafir mindset gives this impression that, even at that late age, they are still "kids", and this in turn causes them to have an immature, childish mindset and causes them to lack responsibility. Causes them to waste away additional years of their life under the idea that they are "kids".
This issue is not fully grounded in the minds of Muslims. That is why, if they find a 15-year old man (take note of the keyword used here) fighting in Jihaad, as a Mujaahid, what will they say? "How can you allow child soldiers!?"
Isn't that right?
But the fact is that only Kaafir society will regard that man as being a "child soldier". In Islaam, he is an adult. This issue of when a person comes to be regarded as a responsible adult affects many other things. Adults have to take responsibility for the consequences of their statements and their actions. For example, and this is an example which those who have grown up all their lives with the Kaafir mindset will struggle to accept:
If a 15-year old commits Irtidaad (apostasy), he is to be killed.
It goes even further than that:
Let's say he had become Baaligh at the age of 13. Now, he commits Irtidaad or does some action which requires the death penalty. Despite being 13-years old, it will be carried out on him, because in Islaam, he will be regarded as an adult and not as a child. He is Baaligh. And again, 15 is the cut-off age. No becoming an adult later than that.
So those who are that age and older should realise that they have been adults for a long time already and thus should act accordingly.
والسلام