I got my first diapers when I was 14, none of which I actually used. A cloth diaper, which I pinned to the back of a pair underwear I cut up, and had the front of a diaper, and a disposable that I would stuff in my underwear.
At 18 I bought my first, baby diapers that I taped together to fit me. These I wet on the toilet in case of leaks, which happened.
At 22 I bought my first adult diapers, and haven't stopped since.