It's really up to you and what you want to get out of the experience or what you feel comfortable with handling. I was originally planning to get an adult or yearling snake as my first but when I went to the breeder I ended up really liking one of the 4 month old babies so that's what I ended up with.
Pros and cons of getting an adult snake:
Pros
- Won't need to go through figuring out what size prey they should be eating as they grow or transitioning them from mice to rats...etc.
- You get what you see -- the colouring of adults won't change
- Adults will have a more established personality, whereas babies can start off nippy and may or may not calm down or change as they grow
Cons
- Don't get to watch it grow up and change
- Babies are cute
- Don't know the background of the snake (its experiences with handling, any bad experiences it might have had)