If a 14 y/o old girl wearing a T-shirt that says “Boy’s Lie,” thinks the apostrophe is correct, is she dumb, or too young to understand?

Answer 1

Not dumb or too young ... just not well educated.

Answer 2

Odds are she's not smart enough to recognize that as a misuse of an apostrophe.

Answer 3

The apostrophe implies lie is a noun and the lie belongs to one boy.

Omitting the apostrophe implies lie is a verb and that all boys lie.

I would ask the girl what she thinks her shirt implies.

Answer 4

If she is making a statement for a lie told by a particular boy, it could be correct, but it is unlikely she produced the t-shirt.