To do with my son I used to just joke with with him about it. Whenever he 'challenged' me with his latest thing he'd seen on playstation or at school I'd playfight him and then shake my head and say 'mmm not ready'
Whenever he got serious and asked or rather jokingly threatened he was going to do Aikido I'd say 'mmmm, when you're ready.
He started at 24 years old. Just thought I'd share it anyway.