My HIB is under a year old, so I can't speak from experience, but I can think of one advantage to buying an adult animal: Infant mortality can be high in any species. If it made it to adulthood, it's already got past that scary baby age. The breeder or owner should also have a pretty good clue about its personality and health. If you know and/or trust the breeder or owner, you're good to go. If not, then you won't know until you meet the animal, and perhaps take it for a vet checkup, plus quarantine.
Case in point: The man I bought my Hogg Island from showed me a lovely Lipstick Sunglow Albino that he bred, which came from Tom Burke-bred parent snakes. Since I know of Mr. Burke and his work with that kind of boa, and since I know Michael Beach (the breeder) in person, when he told me how tame the snake was, and how healthy, I took his word for it, even though I hadn't seen the snake, yet. Upon meeting it (it's a '10), although it hadn't been handled except during cleaning and feeding, it was tame and curious. The snake was big, shiny, healthy, and obviously well cared-for. Needless to say, I bought the snake
.
On the other hand, I went and looked at a coral albino boa the same day. The little guy had no history, it was small, dehydrated and skittish, even though it was the same age as the other boa. He didn't have that look or feel of good health, and the owner wasn't the breeder, nor did he know the breeder or any of its history. I did NOT end up buying that snake, preferring, instead, to purchase the sunglow from MB, who I know and trust.