At what age do they stop growing?

[Gio]

"The number one mistake most novices make is over-feeding. Boas evolved to sustain themselves on a minimal amount of food. You need to guard against over-feeding and obesity. Of course you don't want to starve your boas either, and the goal is to maintain a healthy weight. A young boa will thrive on a single meal that leaves a slight lump, once every ten to fourteen days. This regimen will produce a slow but steady growth rate that the animal can maintain itself on until adulthood." - The Complete Boa Constrictor by Vincent Russo

Exactly right. It is actually "almost" impossible to under feed a boa constrictor. There are many different views on feeding, but the accepted, healthy way to go for an adult boa has been stated.

I have given my growing boa meals that have been too large but not as a habit. The key when doing that is to recognize the snake will need a long break from eating. My boa has not eaten since July 27th. I will probably wait until September to feed him unless he is constantly hunting for food.

In the winter, adopting a seasonal light and temp drop can have your boa off food for most of the winter.

Does that sound bad? Well, also from THE COMPLETE BOA CONSTRICTOR, you see Vin Russo mention that boas will grow more in the period they are not eating than they will if they are in a constant state of eating, digesting, eliminating and shedding.

If you have a 5 year old adult male boa coming your way, DO NOT feed once a week. Find the appropriate sized prey and feed like the others have mentioned above.

Strong, muscular, lean, loaf shaped boas live the longest. "The boas that live the longest, get the biggest." (words from Gus Rentfro)