Not if I don’t need to, like if I need to have a conversation with someone that doesn’t need to be overheard. In any case turning your phone off and putting it in a faraday bag then putting it somewhere relatively noiseproof should be more than enough if you need to bring your phone with you.
I don’t think there is a “dead giveaway”. Plenty of kids can pass as adults online and plenty of adults seem like kids online. And sometimes with stuff like word usage/grammar/etc you can’t tell if it’s a child or someone who doesn’t speak English very well or maybe an English-speaking adult who happens to type like that. There’s a lot of different people in the world.