I've often wondered about this as well. And, I wish there was an answer!!
My son had his first flare at 16 1/2, diagnosed just before his 17th birthday. But, while it's accepted that pediatric crohns can extend and develop complications, I don't think '18' is a magic number. When you get to 17, 18, 19, I think it becomes a bit more 'grey' in determining pediatric or adult crohns. I think the reference to '18 & under' is a reflection of the
average stage of developement of the entire population. While my son has certainly shown developmental/physical changes since then, at diagnosis he was already fairly mature - 5'10", full beard, emotionally mature, etc. While he was still clearly developing, he at the tail end of puberty/development, I think he would fall more closely as being diagnosed/presenting as an adult (when compared to 12, 13, 14 year olds).
Biologics are a great med (my son is on remicade) but, other meds, such as imuran, can be just as great if they're working. If you and your GI have found a med that is working, I wouldn't change just for the sake of changing. There are many people who successfully maintain remission on imuran for years (and there are people who find that biologics fail them).
My son has not had a flare since diagnosis and, for the first two years, my son maintained clinical remission with his only treatment being exclusive and then supplemental enteral nutrition (no meds). So, age does not always determine early progression. We eventually added remicade (3 years ago) because the supplemental EN wasn't clearing up all inflammation and GI worried about future complications if we allowed inflammation to simmer.
But, i do worry about progression and worry about his next flare - when, how will it present, etc. Unfortunately, while I'd still love to find that crystal ball!, I think I've accepted there is no 'norm' that can be anticipated. :voodoo: Crohns is such an individual disease, how it presents, what it affects, how it changes, how you respond to meds, etc. that no one will be able to tell you what will happen in the future. I've thought to ask my son's GI this question but haven't because, in a nutshell, if he tells us he's unlikely to flare again given his history, my son (and me) may become complacent with diet/supplements/meds, if GI tells us its only a matter of time, it'll be a constant cloud of worry, taking away today's pleasures... so, I just don't ask and try not to wonder about it too much.
I know I haven't exactly answered your question but maybe it helps to have a different way to consider this question?? :ghug: