I know that in the first book, they have already left Hogwarts and have already established careers. Well, I guess Charlie could still be in post-secondary school, as Ron says he’s studying dragons in Romania as opposed to working with dragons. Bill is already a curse breaker for Gringotts, so I’m guessing he finished whatever additional training was needed and is into his career.
My guess is that Charlie is 18ish and fresh out of Hogwarsts and Bill is maybe 20ish? Allowing for 1½-2 yrs between kids.
Any other thoughts? Does anyone know if Ms. Rowling has commented on this?