A lad, being another term for a boy, is not opposite for anything in particular. It isn't opposite of "man", "lassie", "girl", "woman" or even "baby" because there is commonality among all of those terms. Different is not enough to make opposite.
Some terms have no clear opposites, some do. Good and bad are terms that are opposite because there is nothing in common. North and south are opposite directions.
If you cannot pair the term with one, and only one term, there is no clear opposite. If you say that male is opposite of female you can do so only by overlooking those organisms that are asexual and those that have both sexes in the same organism.
For those terms with no clear opposite, the proper answer is "Not that". The opposite of "lad" is "not a lad".