According to Duolingo, "when I was a kid" in Spanish is "cuando era niño" or "cuando era niña". Google Translate says the same thing.
That sounds strange to me. Being a kid is a temporary phase, not a fundamental part of who someone is, right? I would think it would use "estar", not "ser", and that "when I was a kid" would be "cuando estaba niño".
Do Spanish speakers use "era" or "estaba" for this? And if they use "era", why?