I'm pretty sure that Axel said/wrote/implied in Days that he turned into a Nobody as an adult, and that's something that set him apart from Roxas. But in BbS (supposed to take place 10 years before KH1) Lea and Isa are kids. World travel shouldn't been very common back then so either:
1) they were evacuated to Traverse Town and grew up there until for some reason they fell to a heartless attack (we've seen it happen to a random NPC in KH1, so it's not impossible);
2) they found a dark path amidst the chaos and escaped through it to some other world; Isa might have been exposed for longer than he should, explaining Saïx's eyes (and putting a new twist on Axel's warning to Sora in KH2 right before his death).
Braig might simply have been at an age where he suddenly started to get lots of gray hair, and maybe a few more scars would make him look older, but Ienzo remains a problem to me, unless the theory that Zexion's appearance is an illusion holds.