At what age do they stop growing?

[skinnyrascal]

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)

Okay I gotcha. I have a care guide on boas but it's kind of vague when it comes to how much and how often you should feed. I'm not desperate for a huge snake, I just don't want him to be stunted and underfed. I'd aim for strong, healthy and average.