Make sure that you always change your birthday, even if it’s a wrong one, to 13 years of age or older to be able to change it at a later date. How to change your age on Roblox account (PC desktop)?

Change Roblox username or Roblox display name When you initially sign up to play Roblox, you must give your Roblox account a Roblox username. Other players will see your Roblox name in game.