Officially, Roblox is available on multiple devices, each with it's own age rating. There's also a lot of gaming content rating systems with various resources to look up details on specific games:
They also have a section on their website for parents here, which talks about the various systems in place to protect children.
That said, Roblox is a game focused on user-generated content, so it has the same problems that any game with online features runs into.
First, it's impossible to have this be a perfect system, so it's always possible that something inappropriate either slips through the cracks or has yet to be flagged and removed.
Additionally, Roblox has little to no control over related content posted elsewhere. It's possible for there to be inappropriate Roblox content posted on other websites that a child might stumble upon from searching the web for Roblox media.
*Amazon defines this as "Based on information provided by the developer, the content of this application has material that is appropriate for most users. The app may include account creation, location detection, user generated content, advertisements, infrequent or mild references to violence, profanity, or crude themes, or other content not suitable for all ages."