My first MMO was actually runescape, and this is when I was in grade 7 so I was 13, 7 years ago. It was 2D back in those days and while primitive, it was fun to log on with a bunch of my classmates and run around questing (single player quests, that we just finish at the same time, lol), finding dungeons, and searching the wilderness for noobs. I have a nostalgia for that game, even though it is horrible by todays standards, I do feel it is a good introductory MMO, for kids just getting into MMO's.
My second MMO I shall mention because it was a bit different than most peoples 2nd MMO, it was Puzzle Pirates; a pirate game where every action you did was a puzzle, so it actually made you think, and what would normally be grinding (let's say, bilging or sailing) was actually a fun puzzle. There were trials and it was also pay to play, and my buddy paid for me and we actually had one of the strongest crews on our ocean (servers were called oceans). I was really good at sword fighting and he was really good at navigating so I think we just had a natural affinity to creating a strong crew. I have fond memories of that game as well, but at the same time, today the game feels like a complete mess to me with all the additions they made, but I'll always have my nostalgia.