I love this question, or should say, love answering it.
First trip I was 10 years old, and went with parents and siblings. That was 1986, so it was just MK and Epcot. Had a good time, but wasn't in love with it yet.
Second trip, I was 14 years old, and again went with parents and siblings. That was the trip I really fell in love with the parks. Sadly, it would be another 6 years until I got to go back.
Third trip, I was 20 years old. My friend calls me one day in mid February and says he's going to WDW with his parents and asks if I want to go next month. Just need spending money. I say yes, we go, stay on site for the first time, and fall in love with the resort. Since then, I can't get enough of it. No matter how tough I can be on the place and no matter how much I expect of it, I know I will always love it.