Ageist

Someone who discriminates against old or young people.

A: Hey let's bring this new born baby on this roller coaster
B: Nah dude I think he's too young.
A: Stop being Ageist
Or
A: Grandpa I think you should retire. Your in your 90ds and you've been working for 70 years.
B: Stop being ageist, I'm fine.