They are usually sold at about a year old... Unless they were bought as baby bettas, then 4-8 weeks. As long as he is strong and active enough he can breed. Make sure you can find the fry homes... And most importantly have time, patience, and take in mind expenses.
Most are free swimming now. There does not seem to be a lot - which is why I removed the father yesterday as I did see him snacking.... -.- so, he is a half-decent father. I don't see having more than 50 ATM.