Some alters may have originated out of a traumatic medical event, so it would make sense if that alter were triggered by a similar medical event.
Age regression is a known phenomenon in DID and stress may certainly trigger this. Some of my adopted son's alters appear to slide among various young ages. If you think about it, age regression would be an accurate description for what happens when any adult alter switches to a younger alter.
Alters can present as older than they are, especially during non-stressful times. For us, some alters can lock onto a vague "adultness" so that they can act, talk and appear to think like adults. But if they're stressed or triggered and that adultness gets detached, they appear and sound their alter age.
Sometimes a young alter routinely pretends to be an adult or at least older. Stress can cause them to lose that pretense.
There's another phenomenon where a young alter is able to connect to certain
procedural memory, which is knowing
how to do something. This can make them seem older than they are. Soon after our young alters awakened and joined us, most were able to turn on/off lights or do some complex tasks they didn't learn themselves.
We've sometimes assumed a younger alter is the same one as an older alter, just regressed, when it's actually a different young alter switching in. In our system, these two alters are more connected than another two random alters. So when our Carter first visited, he felt just like me -- John, the host -- regressing to a younger age. That was because I happened to have replaced him as host when we were six so we share a deep connection -- I was sort of formed out of him. His emotions still feel more like "me" than some others and can overwhelm me more than the emotions of others.
Sometimes we've just misguessed an alter's age, typically assuming they're older than they are.