@Loh Jane,
Quote:why can't I say 'while I was a kid'?
I'd say that you "can't" say it, Jane, because it's not the natural choice for a native speaker, which I gather, is what you would like to be.
It's one of those odd choices that seems very natural for many situations but for some odd reason, in certain circumstances, it's just not used.
Michael Swan in
Practical English Usage, page 74, explains it this way;
Quote:2 Simultaneous long actions while;as
We prefer when to refer to ages and periods of life.
When I was a child we lived in London. (NOT As/While I was a child ...)
His parents died when he was twelve. (NOT ... while he was twelve.
[Do you have access to this book? It's excellent for sorting out these small nuances.]