When is a betta fish to old breed?

There's no set age limit for how old bettas can survive but they age significantly between two years of age. For breeding, most Betta females are bred between the ages of 4 to 14 months when they reach their peak egg production.

Male Betta can breed any time after they're mature enough to build bubblenests, as early as 4 months. Males have less impact by time on breeding quality so you can successfully spawn them years after two, but the breeding performance of the betta can decrease with age because sperm become increasingly scarce and low density and less vigorous which could result in poor fertility rate over time. Male bettas reach their 'ideal' age for maximum fry at two years. So breeding with old bettas that are past 3 year olds might start showing lower than normal fry rate which is why young females and males are preferred.