While a Steam account is supposed to be personal and non-transferable, it obviously isn't so in practice. So unless you provide additional information to prove that the owner is still the same person, then it is probably not sufficient to prove the current owner's age.

Looking at the "marketplace" G2A, about 15% of the Steam games sold there are sold in the form of accounts: Instead of a key or a gift, you get a login and a password for an account owning the game being sold. Making the account age a form of identification would no doubt also create a market for aged accounts without games.