Well certainly school age would be amongst the most difficult time to deal with incontinence, and bowel would be worse certainly. As for an easier age, well, physically it's never easy to deal with, and actually I would assume it would be physically more difficult for an older person to deal with. However, on a social level, it would be easier to deal with as an older person.
I'll put it like this, a younger person likely would not need assistance in changing their diapers, disposing of used ones, or procuring them from a store. Myself for example, it wouldn't be all that difficult to deal with on a physical level, I can easily change, buy them from a store, dispose of them, etc. However, socially, I'd say most people would expect an older person to become incontinent as a sign of old age, whereas for a younger person, it's not as common. Add that to the fact that someone in school would likely stand some ridicule, it could be difficult for sure.
Also, degree of incontinence plays a major role as incontinence is really measured in "degrees", not an all or nothing type thing. As for me personally, most of you already know, but I deal with an extremely mild urinary drip and I'm young-ish. It barely affects anything, though it does require me to wear a guard during the day. Nobody knows, it's extremely discreet and not an issue in the least. If it was more severe, and I'm glad it's not, well then I'm sure it would force me to have to make some lifestyle changes. So yeah, that's my opinion.