ME: I can drink three strong cups of coffee every day without any negative effects! ALSO ME: I wonder how I got this vicious insomnia, which is getting worse.
@MitchW I'm almost completely immune to caffeine at this point, even though I didn't start drinking coffee until I was in my 50s. In my younger teens, I drank nothing but instant iced tea at home.
@nitpicking Did you consume caffeine between your teens and when you started on coffee? What made you start drinking coffee in your 50s?
I started it in my teens. It felt like a very grown-up thing to do. My parents and most of my aunts and uncles rarely drank, but they consumed gallons of coffee.
@MitchW I just realized I never answered you, sorry. I had lots of caffeine between my teens and 50s, just not 100% of my at-home liquid consumption.
I started drinking coffee because it has (probable) health benefits and is cheap (when you brew it yourself).
@nitpicking did you have difficulty acquiring the taste? was coffee an unpleasant flavor for you at first? How do you take it, black or with cream and sugar? 
@MitchW I had no problem acquiring the taste. Black, with artificial sweetener (1 "dose" per cup). I like the mouthfeel of black coffee.
I've never been a liquid dairy guy.