I think the optimal size for a female should be a cross between weight and length and not so much age. I bred my albino burm for her first time this season. She was 9.5-10 ft and a whopping 40 lbs and 3 years old.
This was also the males first year. He was barely 8 ft and im not sure of his weight and he was also 3 years(yes hes a little guy) and they both ate all through breeding(until the female became gravid) which was a very nice supprise.
Heres what I go by.
Females minimum 9-10 ft and males about 7- 8 ft minimum. There are some cases where males are'nt quite sexually mature at 7 ft, but I believe most are. Mine was.
Just make sure your female is FAT and your male is a little bit on the thin side of HEALTHY. This will make him a more ambitious breeder. Fat males tend to be lazy