[muzani]

"Woman" sounds a bit old, "girl" sounds a bit young. "Guys" (or "you guys") is slightly gender neutral, so many people would use "guy" as a gender neutral term, and avoid "gal" altogether.

In my native language, there is a semi-formal term we use which is neither rigid nor offensive... "tuan" for men (Sir) and "cik puan" for women (Miss Mrs). For English, I think the closest to this is "lady", and this might be why many non-native English speakers use "sir".

[gshdg]

What’s old about “woman”? “Woman” means “adult female human being”. That includes 18-year-olds.

[muzani]

It should, but that's just not how society uses it. A google image search doesn't turn up many 18 year olds.