I don’t think there is limit on buying bowling balls. If anyone goes to a pro shop to buy a ball, the shop will not turn you away. I have seen kids as young as 6 or 7 go to pro shops and buy the things that need so I don’t why there will be a age limit. I would say that kids should have a parents with them to buy the ball and should be with them at all times when buying anything. But I don’t think there is a age limit on buying bowling balls online or offline.