Betta are bred from 3 to 6months. By 9+ they are full length but are still putting on weight. At 1 year, they're full grown and too old to breed. Their fins continue to grow, however slowly, throughout their lives.
If a petstore fish is older than 4 months, the breeder has lost money by feeding it, housing it and keeping it too long. That's why they try to ship as soon as they can....including the pernicious "baby" Betta trend.
Yes, there is a "stunting hormone" excreted by fish trying to keep other fry small and uncompetitive.