Again, you're misunderstanding the nature of privilege. Having the choice of whether or not to have children at — or after — 18 is a privilege. There are parts of the world (arguably, even parts of the US) where young women don't get that choice, whether through (lack of) education, (lack of) access to contraception, forced marriage, or otherwise, those women have had that choice taken from them.

To have the choice is the privilege, not whether or how it's exercised.