Roblox is designed to cater to an audience of all ages, though its notably played by younger gamers. As a matter of fact, 67% of Roblox players are under 16, while only 14% are over 25.
Despite the disparity, please remember that Roblox, more than a game, is a platform where everyone can create and share their own games. As such, its expected that players could meet players of every age and walk of life. This includes games that could be violent or explicit in concept and design, though Roblox does not allow graphic content outright.
Nevertheless, if security is a concern, parents can adjust the content their children can access through the parental settings in their accounts, as well as secure these settings via PIN.