Just to add to what Randy said, I think early imprinting occurs when being handled, and has a life long effect on them. The ones I've had that came to me as young adults or older have always been flighty, nervous, or somewhat aggressive about it.
All the ones I've had since they were neos, grew to be calm and accepting to handling. My large albino female Aurora LOVES (apparently) to be handled. I can get her to coil in my lap and just sit there.
I'm even getting this little chainsaw Honduran girl to settle down and not try and eat me all the time. (Notice she leaves herself plenty of room to strike if she should take offense to something I'm doing.
)
Point is, even the little biters, if handled properly when young, can overcome their bitiness, and become pretty calm snakes as adults.
Always keep in mind though, that Boas are not tame, and very instinctive. They are ALL capable of biting, so awareness is a must when handling. But mostly, it's not a problem Boas tend to have if raised properly.
Ain't a better animal alive BTW. I've had them since 1975.