Mine, if I remember correctly, was gradual. As far back as I can remember, I liked feeling cozy by lying on my bed, tucking myself in, lying on my tummy and placing my hands under my thighs, palms up.
One time, when I was 14, I was doing so and felt a novel awareness of my penis and groin. I started thinking of a girl (none in particular) having a laxative, and imagined her speaking, such that she would replace short "a" with soft "a", such that "laxative" becomes "lahxative". I squirmed as would sometimes do, but this time was different: This time I felt as though I had urinated, slightly. I repeated this every Sunday afternoon for several weeks, before learning what it was, by doing my usual science-related reading. At about age 17, it occurred to me that if I masturbate before seeing my girlfriend, my celibate dates might involve less stress. It worked out noticeably well.