Check if player under 13?

So I'm trying this script:

function GetUnder13(player)
	wait(0.01)
	game.StarterGui.Opening.Warning.Visible=true
end

It's to give players under 13 a warning upon joining the game but it's not working, I've tried in a local & a regular script. If anybody knows how to get this to work, that would be great. Side Note: GUI Info: V Game V StarterGui V Opening V Warning