Monetizing my hobby at a young age.
12 year old me realized that all the cool gaming groups had websites, so I figured out how to build one for me and my friends as well. At 14 I was providing such web development services through a gaming Internet forum (primarily World of Warcraft private servers). After a while, I decided not to reinvent the wheel for every customer, so I built a web platform called FusionCMS together with my then newly met Internet-friend Xavier.
We sold nearly 300 copies and even provided a third party marketplace of themes and plugins. We were never motivated by the money, but by the technical challenge. However, looking back I consider this to be my first business. Apart from just the programming practice, this side project taught me plenty about everything from public relations to customer support and taxes.