I'm posting this from a 7 year old phone with an OLED screen. The screen still looks as good as the day I bought it.

I am worried about burn-in on computer screens, but at the same time I am just wondering about how others use their phones, my last 4 phones had OLED and I have never had any burn in occur. I bought a used Galaxy S4 mini at some point and when I got it had slight burn-in of some icons, but it didn't get any worse in the two years I was using it. Am I maybe just too old because I use a computer while young people use their phones for 10 hours a day?

I cannot confirm. The phone I'm writing these very words on is a Samsung Galaxy S10 Plus from 2019 running LineageOS 20, and the AMOLED display is absolutely gorgeous and looks as good as today's top-tier smartphone screens. But maybe that's because this is a Samsung flagship, and Samsung is notorious for making kind of the absolute best displays for their flagships.