I remember firing 12 gauge shotguns and an 1870s Springfield 45-70 as a boy of 12. Didn't scare me and it was fun!
One year I needed a rifle for deer hunting and a friend offered to loan me his 30-06. I had shot a 30-30, shotgun with slugs etc. so I went out and bought a box of 20 to sight it in. We went out to shoot it and said something about shooting the whole box. After about 4-5 rounds, I was done. It kicked my ass but I did get a nice deer that year from quite a ways away.