Can 男の子 be used to refer to a young adult male? The character they are referring to is 18-19 years old. Two others characters, who are 18, are also referred to as 男 (adult male).
Does it just emphasize that he's young, but not mean that he's literally a child?
The exact age limit for the terms 男の子 and 女の子 would differ person by person, but the general consensus is that anyone under the age of 30 and unmarried can be referred to as 男の子 and 女の子. Here, the 子 simply indicates someone who is still young, and possibly immature. It should not be literally understood as a "child."
Once a person surpasses the condition of being called 男の子 and 女の子, they can then be referred to as 男の人 and 女の人. Please DO NOT refer to anyone as simply 男{おとこ} and 女{おんな}, although it literally means "man" and "woman", it is a very rude way to address people. Always use 男の子/女の子 or 男の人/女の人.
In formal context though, we refer to "males" as 男性{だんせい} and "females" as 女性{じょせい}. You will likely see this a lot when filling out official forms. For 男性/女性 there is no age limit.
Don't take the 子 part of it too literally, what you should hear is the casualness. It is not comparable in all ways to these English terms, but it has somewhat similar timbre to "guy" or "dude." A certain amount of youthful immaturity is at times implicit (although not necessary). There is a tinge of status disparity possibly felt embedded in the word in that the person speaking the words probably is older or is in some way in a higher social stratum (depending on context and addressee), but this is not always the case. A neutral way of saying male is 男性.
