1978-79, Monochrome Blue boxed set, parents, toy store? book store? not sure.
We (my brother and I) tried desperately to get our parents involved (we were 6-7-8 ish and had no friends), but ultimately that didn't work out and we were left to our own devices. Very confused when the Dungeon Master's Guide didn't synch up well with the material in the box (just how advanced WAS this "Advanced" thing anyway?). Then the Monster Manual continued to confuse us. Only upon getting the Player's Handbook did we fully leave OD&D for AD&D1E, and I, at least, have been "in" ever since.
Though only primitive reading and toying around with ideas until I hit 5th grade and found fellow gamers for the first time. I still remember buying an adamantium (yep) castle in the Astral Plane (from a classmate) that was stocked with a "zoo" containing the monsters from the S4 insert. All of 'em. Even the demon lords. But my wizard was 500th level so it was no big deal. It was only a few million platinum down the drain; what did I care? I could always make more. So I remapped the thing (figuring I could both reconfigure and redecorate my new digs with very little effort), placed it much closer to the Abyss/Nine Hells/Astral boundary and made myself responsible for holding back the tide of fiends that was always poised to invade the Prime.
In hindsight, that character may have been a little over the top. But when I think about "how did I start playing," I have to confess to this character. I don't, however, have to repeat his horrendously cheesy name.