Having once been involved, albeit in a very minor capacity, in the assessment of F/Os approaching captain, I noticed that the major factor in the RHS to LHS transition was the mental transition. There is no doubt that anyone can fly the aircraft equally well from either seat but pre-command training comes in to it's own when the candidate is given the opportunity to be exposed to some of the off-the-cuff stuff that can occur, particularly down route, that requires the bit extra that captains need to have sewn up in their back pocket, thinking outside the square, as it is sometimes called.
Given the opportunity to experience some out of the ordinary experiences from a captains point of view and given the opportunity to make the decisions, does wonders for improving the aspirants confidence. Should the supervising captain choose to vary the decision then he must explain why, in detail.
Captains can be trained to be captains BUT the basic material must be there in the first place, if that material is missing, such as the ability to make decisions based on all the information available, the ability not to get too flustered in non-normals situation, etc. etc. but to hold the whole flight together, follow the checklist and make decisions based on common sense, the aircrafts ability, the facilities available at your chosen destination, utilisation of the entire crew to the best advantage etc. etc. and on, on, the list is endless etc. that is what a captain has to produce to a greater or lesser degree. Some of the above can be taught but not all, some of it has to be innate and be natural, only some of it can be gained by experience.
It is a myth to assume that a command failure is the company's fault, there are people out there who won't make it, there are people out there who would make it under a less demanding system, bigger budget, more demand and less supply etc.
Dealing with an emergency of any sort on an aircraft half way across the Pacific at the dead of night is not likely to be the same as dealing with a similar emergency half way across Europe but a captain who has served his time on both domestic and long haul and then gets his LHS long haul should be, by experience, reasonably well prepared.
My point? err.......... well yes, nothing wrong with a young captain just so long that he knows his stuff, his aeroplane, has innate command ability and consequently the respect of his crew.
As the Middle East and Fragrant Harbour forums will show the idea of what is a reasonable time between gaining one's licence and gaining ones first command vary a whole lot. I have flown with people who, given the time and training and money, would eventually make captain, I have flown with people who would never over come the 'pressure' of the added responsibility but would forever fly the aeroplane beautifully.
Command is not a Divine right and not all aspects can be taught, but, generally, the greater the age usually means the greater the experience!