My first diaper change? I really can`t remember, as I think urine would be included in that - but the first actually nasty diaper I changed of someone unrelated was when I was babysitting for my neighbors across the street at 10...
Beyond that - I had late siblings. A sister born when I was 8, a brother when I was 14. And a mother who had no sense of responsibility, especially with my brother. I did pretty much everything, of course including the diapers. And eventually I had my own child.
It`s also, strangely, different when it`s your own baby. As sick and weird as this sounds, it is medically documented that the odor of your own child`s waste is nowhere near as offensive as that of another baby. Plus, you get the amazing advantage (if you`re breast feeding, which there is pretty much no reason you shouldn`t be) of having the difficulty level go up in steps. Newborn feces is almost odorless...
It`s better to just focus on getting the task at hand done and over with than being silly and making a big deal about how disgusted you are. It just takes longer that way and you give the baby more openings to make the mess worse.
And if you can`t handle a diaper, just wait until your baby gets the stomach flu and projectile vomit comes your way.