I am approaching my 66th birthday and am not in diapers (yet). Women who have carried babies to term have more risk for weakened sphincters in general, so that puts them at higher risk with a j-pouch.
Any person who lives to be old enough may face wearing diapers as they age. I would suggest you not worry about things that may not happen. As for myself, I really cannot imagine getting an ileostomy unless I had full incontinence with zero control. Even then, it may not be medically feasible at that point, depending on my overall health and condition.
Jan