They could help as soon as they are mature enough to work with the equipment, and materials, and as soon as they are old enough to have the dexterity to handle stuff too. Maturity will be an issue too, as they will need to be able to follow instructions well.
Personally I don't think it's a good beginner project. There's too much risk of a tank leaking, and that's not something I would think a beginner would want to deal with. |