For me, it was June 2001, and I was 17. Before then, I was aware of adult diapers from places like the Sears Catalog, and had known about Goodnites from magazine ads. I had been following the Goodnites online forum since around its beginnings in approximately 1998, looking for signs that other teens were like me, and wanted to wear diapers.
In June 2001, there was a breakthrough. Someone posted that their child was purposefully wetting their Goodnites because they liked them. Among the responses to that post, was a link to the website "when kids love diapers" (or WKLD). I visited the site, and was so happy to learn that I was not alone. From that site, there was a link to DPF, where I learned a lot more about ABs and DLs. It still took me a few years to be brave enough to buy my own diapers after that, but since then, I have come to have a greater acceptance and enjoyment of my DL side.