My 6 year old asks to play Roblox every waking hour!

It's been a month of saying "No" each time he asks if he can play Roblox. His cousins (10 and 12) play it, and so do a few kids in his kindergarten class. It's beyond annoying at this point. A part of me just wants to give in, but I don't know for sure at what age is Roblox appropriate. He specifically LOVES cars, and wants to play the "Driving empire" game in Roblox.

Wondering if others have faced a similar issue and how you handled it with you kiddo.