It's only a matter of time before it gets screwed up.
Then please show us the reports of these adults between 18 and 20 with CCW permits screwing it up. This has been legal for years in other states (off the top of my head, Alabama, Maine, and New Hampshire have minimum ages for CCW at 18) so if it was a legitimate problem, there would be ample evidence. This comment seems very much like the old "there will be blood in the streets" arguments.
I think it's interesting that people who are otherwise pro gun right expansion (not singling out anyone in particular) are uncomfortable with expanding those rights to a class of people they don't like.
Several states have had permits for anyone over the age of 18 for years and there haven't been any issues of teens making bad decisions and shooting people who shouldn't have been shot. At least no more so than the general population. Maine, New Hampshire, and Alabama all set the minimum age for a permit at 18, and there are even more states that allow open carry at 18 without a permit. I may be a bit biased as I was one of those 18 year old "kids" with carry permits from 3 different states, but I feel that if someone is going to go through the time, cost, and effort (and maybe training) to get a carry permit and do everything legally, they're probably going to be ok carrying a gun.
To take the age restriction line of thinking to the other extreme, I know of a lot of older people who legitimately shouldn't be handling firearms, or for that matter other dangerous equipment like chainsaws and cars. There is a significant portion of that population that has either Alzheimer's or dementia. If mental maturity (and not age of majority) is a criteria for determining who gets constitutional rights, it should be equally valid at the other end of the spectrum.
Therefore in the spirit of A Modest Proposal, I suggest that we restrict the firearms rights of that group as well. Really, we don't need a bunch of old people running around with guns, they'll just shoot the neighborhood kids for walking across their lawn!
So logically, your constitutional right to bear arms should only apply when you're between the ages of 21 and 65, can I get an amen?
You may also recall that the NRA recently fought to overturn a ruling similar (but much more limited) to my hyperbole above.
As far as training, any good training program (regardless of target audience) should espouse the benefits of a risk adverse lifestyle. The age old "don't go stupid places with stupid people and do stupid things".
I also think force on force should be more wide spread. There's nothing quite as sobering as doing a training evolution where the bad guys shoot back. It makes people quickly realize that the gun is not a talisman and they are not a superhero.