We were talking about under 21. You keep saying “kids”.
How about a 20 year old with a .357 with heavy trigger pull.
Just out of curiosity, how many .357 autoloaders have you seen?
Or did you mean .357 SIG, which is an autoloader, but pretty rare.
Most of the .357s in the world are revolvers or single shot, although there are also rifles, primarily lever action made in that caliber. I have no problem with 18-21 year olds with any revolver they want, although I suspect that they will soon learn that the .44 magnum and above are not really intended for everyday carry or frequent trips to the range.
A revolver is the handgun I always start learners on, although in a lower recoiling caliber at first.
Now if you want an example of a non-autoloader I would never let any younger shooter use without a lot of adult supervision, it is the good old .30-30 Winchester. A lot of people buy one for their kid's first deer rifle without ever stopping to think that the only way to unload it is to jack every shell in it through the chamber, one at a time.