I'm wondering what would be the right way to say "the club is full of girls/women" if these girls/women are between let's say 18-25? Might filles work or is jeunes femmes more adequate? And what about if one is talking about boys/men of the same age? To me the word garçon seems to primarily refer to younger boys. I guess mecs might work but maybe there's also a less familiar way of referring to males in this age range?
Jeunes hommes is used quite naturally for non-maried men of this age. Garçons would be said by someone of the same age, hommes would be said by someone younger.
Considering the same range of age, femmes and even jeunes femmes tend to make them older. Filles is more natural than garçons.
As long as we are talking about young adult girls, it mostly depends on the age of the one who speaks, but the choice can also be driven by the context.
For example :
- If one wants to emphasize that a nightclub is full of girls of their age or younger, they will say "cette boite de nuit est pleine de filles".
- If one is older than most people in the night club and wants to emphasize that the girls are old enough to be consenting, they will say "cette boite de nuit est pleine de jeunes femmes".
About "mec", "meuf" and "nana"
Mecs and meufs are much more familiar but also more equivocal about the relative age.
Nana is quite old and not very used by millennials or younger people anymore.
I'm a French man from France. I think most French people would agree with this answer, but opinions on this subject may differ in other countries.
If you say mecs, you can also say nanas although it doesn't really state how old they might be, otherwise jeunes femmes is right for this age range.
You might also say il y a surtout beaucoup de jeunes adultes, hommes et femmes, dans ce club.