This may be too complicated for youth, who sometimes need things to be black and white or absolute.
As Muslims, we like to see Islamic law as fixed and perfect and absolute, an exact copy of the divine will.
However, in reality, the derivation of Islamic law is a process. Some rulings are stronger than others.
For instance, it is obvious that drinking wine is prohibited in the Qur'an. There is 100% certainty on that viewpoint.
However, it is not obvious that a marja' must be a man. This is more of a viewpoint that has been deduced based on assumptions about the ideal social role and nature of women, as well as some sideways derivations from texts. (Probably, the era and cultural environment of these discussions also had some effect.) That is, we don't have an ayah or hadith that specifically says "a woman cannot be a marja'" in the same way that the Qur'an says "do not drink wine".
However, instead of understanding the process of how rulings are made, some Muslims start at the result and go backwards. That is, they take the ruling that "a woman cannot be a marja'" and then, from it, deduce all sorts of things about the role of a man or woman in society. This is a sort of layperson's error that comes from a pious but incomplete understanding of the Islamic scholarly tradition.
So... the short answer is... yes, it is a common viewpoint that a woman may not be a marja'; however, one can also make a case that it doesn't matter if a marja' is a man or a woman. Perhaps, the dominant viewpoint on this may change in the next couple centuries.
This is all theoretical; it is unlikely, in the present day, that a woman will actually become a marja'.
As for how you wish to discuss that with the youth... that is a more challenging question. My personal approach to dealing with young people is to try to be as honest as possible; at the same time, if they are growing up as a minority, sometimes they really need the sense of solidity of everything in their faith being 100% certain and agreed upon, at least up until a certain age. Perhaps someone else may be able to advise on this.
