Welcome to the forum.
No way to really to pin down the age. Snakes grow at different rates depending upon how well and how often they were fed. Even snakes from the same clutches and litters can differ.
You can have an idea of at LEAST how old it is. Being a 6ft (?) I would guess 3-4 yrs. That is just an average.
Now, someone may come along and say, "it took 5 yrs for mine to hit 6 ft", and someone else will say," mine took 2 1/2 yrs". It just varies.
Also, if yours is a male, it MAY be slowing down on growing to maxed out. Maybe another foot to go? Who knows?
If it is a female, she may have another foot or two to go on average.
Boas are a lot of fun. Hey, any snake with a mustache has got to be cool!
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