What’s considered a child?

Not sure what tag this would come under and spoilers just in case

I know it’s stated early on that children and pregnant people are sent to some other facility but was it ever said what’s counted as a ‘child’? Like how old is old enough to be a crawler?

If we went by human standards it would probably be around 18 right, but it’s the aliens running the crawl so is it older, or younger?

As far as I remember (and it’s been a hot minute since I read ddc like a few months or so) Lucia is the youngest crawler but that’s due a glitch we don’t know much about