The oldest girls I have bred were 16 and 11. The 11yr old will be 16 this year and I'm planning to breed her again. I don't breed until they are 5 years or older and breed sporadically, so there's always 3-5 years between litters. Simply 'cause I am lazy and keep for fun. My oldest boa is turning 25 this year, she was the one I bred at 16. She could possibly still be going but I only bred her that one time. She is just now starting to show signs of age, basically she won't eat in shed now but that's really all that's changed.
As long as they are healthy and have good muscle tone, I don't see why your boas can't breed.
good luck!