Apple

Father of a teenager and young adult here. It's more than iMessage; it's the entire environment. iPhones are more durable and supported far longer than Androids and this makes handing down phones to kids a natural proposition. My kids have hand me down phones and my youngest is jealous because my phone has the .5 lens. They share music via the Music app. They know the cameras inside and out. And sure, green bubbles are a thing. But the lock-in is far more important. Parents with middle schoolers who aren't ready to give their kids a phone are now getting Apple Watches with cellular as their kid's first Apple device. Android is in serious jeopardy of being an also-ran in the next 10-15 years here in the US.