Note that I don't write websites. I only have vague clues as to what issues real web site developers face. The above is a very incomplete overview of the type issues a website developer needs to figure out that don't apply to a webpage.
Today websites are common enough that people have figured out all the common issues and built tools. Now a website is easier because we know of solutions common problems and so you can just use the solution.
I eventually sold it and maintained it for 7 years total and then it was abandoned because the site didn't catch on enough, and the game itself assimilated lots of the functionality provided by my in-game database (which did catch on massively with the players) so that successful service became redundant over time. But there you've got it: a website plus ancillary software and services, an entire distributed system, built by one person.
And I knew others who did similar stuff - lots and lots of them. It was very common back then to just write some web app by yourself, put it online and see if someone is interested enough to use it. Much more common than today, as it seems, because today pretty much anything already exists, or if it doesn't, it is probably because there is some kind of legal risk involved that you can't just take anymore as a solo person without a big company backing you up.
Certainly not - any teenager could do it alone even more than twelve years ago. I remember using Notepad - before syntax highlighting was really a thing - to build a site in PHP and hosting it with Apache. The only cost was the domain registration fee.