The first gun I shot was probably a Colt Delta Elite 10mm, or .44 Magnum that my dad owned, at the age of 6. That was with him helping me, and 1 shot in the gun.
The first gun I shot myself, without his help, would also be one of those, at the age of 6. The first gun I shot regularly was a .22 Smith & Wesson.
The most powerful pistol I shot was probably .454 Casull, and I used to shoot it all the time... I loved it. I'm pretty sure I've never shot any pistols more powerful.