Short answer. There is no way. IMDb does not cause age discrimination. People cause age discrimination. Also a thought to ponder. What if a casting director that is not familiar with you personally is looking to cast 200 people that are middle aged, or seniors, or are 20 somethings, and you age is missing, you might lose out on work because the is NO age listed.
If someone tries to remove an actor's date of birth, whether it's their own or someone else's, how does IMDb know the date of birth was correct in the first place? If it can't be proven what the actor's date of birth is, IMDb should remove it if someone says it's wrong.