I didn't know about the UV index or what it meant until like three years ago. It blows my mind I went that long without understanding. It was actually wonderful to learn that I could just see if I needed sunblock based on a 0-10 scale.
We never wore sunscreen as kids because we didn't burn. I probably have had three sunburns in my life. Learned once I became an adult that that wasn't sound logic...
I think the Apple Watch or the Weather app or something finally taught me UV index about the same time. Has this information always been readily available? Similarly mind blown about it.