I'd like to add my two cents here

In my experience, talking to a lot of Japanese men, they refer to women they're interested in as a whole, whether adult or children (because unfortunately I know some interested in children...), they use vocabulary such as 女の子 or 女子. I think the reason for this is simple and the same as in English. When we refer to someone as a "woman", there is a matured and aged aspect or connotation to it, as if the word can make the woman less youthful by description.
However, referring to that same woman as a "girl", or just referring to "girls" in general highlights their youth and, in a non-negatively connotative way, their "immaturity", which (I've seen) is typically more of a preference for girls who want to be perceived as young or fertile, so to speak. Essentially, the embracement of the connotations related to the word "woman" may also entail letting go of your youthful image.
I also want to point out that to me, I have a sense of the "passionate" type of love when people refer to women as 女の子 or 女子. Meaning, they're interested in the girl to "enjoy" them. Another way I could possibly describe it is young love.
Similarly, if I think of women in even in their 20s, they may often refer to men of their age group as "boys" still. It doesn't literally mean that they're talking about underage boys in this case, but it highlights the youth and avoids the mature connotation that comes with such a word like "man". In fact, the only time I hear women using the word "men" is when they have something negative to say about men as a whole, unfortunately.
All this to say, I think these words have very similar connotation rules in Japanese, and I've definitely seen countless cases of 女の子 referring to women over 20 or just women as a whole that people are interested in, and I don't see why that might not be the case for 男の子 as well.
As someone embraces their maturity and old age, at some point it becomes clear not to refer to them as such anymore. I'd like to see if there are any disagreements with what I've said
