Would you refer to a 29 year old as a boy? Then you cannot refer to a 29 year old as a girl.
Actually you can, because “girl” is not only the opposite of “boy”, it is also the opposite of “guy”. We say “boys and girls”, but also “guys and girls”.
In addition, “boy” and “girl” can refer even to old people, if they are servants. This is unpopular in today’s egalitarian environment, but it remains correct English, just as its equivalent in Hebrew is correct and attested to in Tanach (נער).
And then of course there is the usage “the boys”, or “the girls”, which refers to a group of same-sex friends, even if they are quite old. And there is “old boy” or “old girl”, which davka refers to an old person.