My kids use Roblox. There _are_ parental controls you can enable through Roblox. When my twins turned nine, I had to enable 9+ games for them. I believe the age cutoffs are 9+, 13+, and 17+. I think anything with drugs should be 17+, and realistic violence/blood might be 13+? Not totally sure.
(Disclaimer: I'm a Roblox employee, but speaking only on my own behalf, and don't work on anything related to age guidelines.)