As soon as you can step away from the foam one finned floating hindrance , get a longboard have fun, and a whole lot of learning!
Don't even think about anything about when go to a "fun board." All boards are fun
You will know if you are ready and capable to go to another sort of board, when you are ready and capable
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun!