You can get a BP at any age. Babies can be a little more defensive(potentially nippy) since they're so small. They usually do outgrow it though. Older animals are more established so the breeder/owner could tell you what they're like.
If you want a baby, make sure it has had at least 3 meals since hatching. It's sort of a rule of thumb to see if the snake will eat on it's own without assistance. The soonest you can get a baby is probably at a month old if its a strong feeder. And every snake is different. Some switch to frozen right away without any issue. Some take weeks or months. And others will never ever switch. You should ask the breeder to try to switch for you before picking up a baby.