Most people my age learned HTML by just experimenting with it

Why?

Because almost everyone who has been to some high schools and most colleges has learned to either program or talked to someone about how it is done. And if you copy a page (like I copied a page of Yahoo because it was one of the first websites on the World Wide Web that worked very well. So, I copies the HTML of that front page and then changed the programming and ran it to see what it did. I realized my best friend who has a Master's degree from UCLA did exactly the same thing to learn HTML in order to build his own website less expensively than having to hire someone to build it also in the 1990s and early 2000s.

Note; In case you don't know what HTML is it is Hypertext Markup Language which is the main basis of computer languages the users use to build web pages online. It doesn't go as deep as TCPIP in that it is mostly for computers and tablets and smartphones. TCP/IP means Transfer Control protocol/ Internet protocol which is to allow many different types of software beyond HTML to interact on the Internet. It allows diverse machines from home computers to super mainframe computers to interact with one another all over the world now. 

IF you have never seen HTML computer code and you happen to be in Chrome go up to where it says "View" and then slide down to where it says "Developer". Then slide right to where it says "View Source".

This will give you the HTML code for whatever page you are looking at online right then. (or whatever other languages it might be written in like Java for example.