Korin has a good picture and an excellent example. I can only really speak for my self as I am the only person, in real life, which i've met that is, who has DID. But for me, the formation of other members of the system were indeed born during the formative years, and one or two in the teens. Although a long standing history of C-PTSD elements. The cost of living in it today causes a lot of anxiety and panic.

But Korin is exactly right.