Starting with a .22 is always best. The .22 does the best job of introducing people to shooting. Low cost, noise, and recoil are important factors when people are just starting out.
Now, someone who already knows how to shoot doesn't get quite as much benefit from the .22 as a complete beginner, but they do get a benefit.
.22s come in three broad types. The smallest, pocket pistols, .22versions of duty class guns, and "sport pistols".
Shooting a .22 version of your carry gun is fine practice. But shooting anything is practice, and making it fun means enjoyment. Enjoyment means eagerness, and eagerness means you will shoot more often.
Think about this, and while not a perfect analogy, there are some parallels..
Every time you get behind the wheel you increase your driving skills. Hundreds of thousands of highway miles does not make you ready for formula 1 racing, but you are a lot closer to being ready than someone who is just starting drivers ed. For you gamers out there, killing an orc as a first level player may get you 100pts. When you're a 15th level elf ninja assassin that exact same orc is only worth 1pt. But you still get 1pt.
