Thats true on the goverment but maybe they need to start putting an additional endorsement on some licenses that state the driver is a certified trainer.

I look at this in a different perspective...as an instructor/safety officer for a paid/volunteer fire dept, and as a former trainer/Mentor in this industry.

I see 6-9 months for some, a year or more for others....you can't pinpoint a time when someone is ready, or capable of teaching...you just have to take the initiative...when you teach you learn.

Driving as a Professional Driver, I see this as a 5 level "learning curve":

1. Truck school student...you learn the absolute basics to be able to pass the exam with the DMV.

2. Trainee on a working truck...whether it be a mega carrier, a small training company, or a relative teaching you. This stage is getting the actual hands on experience and knowledge you will need to be on your own. Shippers/Receivers, city v open driving, dispatch systems, technical backing instruction, paper work, truck stops, toll v secondary roads, routing, planning, break down...just about everything we encounter.

3. Initial solo...here you put together, and try to keep in your head, what you have learned and practiced for the previous time you were with someone who was your teacher(s). Along with working out the kinks in what was taught you, you learn how to manage your time (and hopefully your finances) and maintain your schedules, your home time, your "work ethic" within this industry. This is truly your first test on how you are going to cope and grow in this industry.

4. Being the trainer/Mentor...now you have to figure out how to explain to someone else, all that you have learned yourself. You won't always be understood the first, second, or even third time you say it...it could take 5,6,or even 7 times to get your point across. It could even take another person to explain that simple task to your student, by saying the exact same thing you said, but with a different tone, a different beginning in the explanation, or just a simple switching of one or two words in the explanation...but it does get done. All the while, as you flounder for a "better way" to explain, you are actually teaching yourself as you are trying to teach another. This takes a lot of patience and humility, and at some point, your patience and/or your humility will run out. Welcome to Level 5!

5. GRADUATION TO SOLO...you are probably still teaching and don't realize it. Here on TTR, at the truck stop coffee counter, helping that rookie get backed into a dock or T/S parking slot....giving advice to another "experienced" driver about a route, a better way to secure a load, telling one of your "tall tales" inspiring that down in the mouth rookie that's ready to throw in the towel. But....you're still teaching!

TheRoadWarrior, Otter, marmonman, all are Great post. If I may reply, Not LESS than 20 years driving experience, should one train. I figure after you put the 20 in, You have been there done that, accomplished all. I might , and might, let the 15 year veteran train, but like the 20 plus year drivers.
Maybe its just me, but Im very comfortable with this. And this is just my opinion, ANYONE under 10 years driving, should not, and is not ready to train.
We could also go the Million Mile way, Not under 3 million miles, should one train. We want the ones that will train a driver to be safe on the road, a veteran driver will know when the new driver is ready. The veteran driver will also, say, your not ready yet, but then the turn over is slowed down, these dirt bag CDL mills want them out in a truck in 3 weeks.

Eh, about 5 weeks seems good nuff....