It's down to personal preference, 42mm isn't a mans size, and 38mm isn't a womans size.
For example, I see quite a few boys (Persons under 18) arms on this forum (and in this thread), who suit the smaller watch, which is to be expected.
For example, I see quite a few boys (Persons under 18) arms on this forum (and in this thread), who suit the smaller watch, which is to be expected.