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?
Although occasionally you can use "estar" associated to expressions related to age, like in:
Estoy hecho un chaval
Estoy viejo
These are expressions where speaker referes to it's personal perception of it's age, not an objective representation of it's true age. How old they feel, not how old they are
When you were a kid, you efectively were a kid, so you use "ser" and the correct expression would be
Cuando era niño
If you wanted to express that in a previous moment in life, althoug being older, you felt like a youngster, you could use "estar"
En aquella época estaba como un crío de 8 años (those days I was like an 8 years old child)
