Zero-knowledge proofs exist, where a person can prove one fact without revealing other facts about themselves. If there were a will to do it, governments could figure out a way for people to prove their age to a website without revealing their identity. The problem is that it was never supposed to be about proving your age, because what governments and corporations actually want is your identity for surveillance purposes. Proof of age and protection of children was just the cover story they used to promote the destruction of online anonymity. The goal is surveillance and control, not protection of children.