On the Internet, what is a dead giveaway that someone is actually a kid?

ASL

I wrote a whole goddamn novel-sized autorespond in Polaris that triggered on an asl. I was a 96 year old tibetan yak farmer who was allergic to wool and hated the local monks. I used to pee in their milk until one day the abbot found out and beat me with his hat.

Either that or it started screaming about asexual lizards

fun fact: a lot of people who don't remember the ASL days are adults now

Your definition for kid is out of date.