Great thread Mike, thanks to my daughter it has taken ages to read, essentially I agree with you, though I didn't really get going with tree work till 26 and properly climbing till 28 when I started on my own. I still think young is best. I had to learn fast because I was working for myself and often with groundies who couldn't climb, so if I couldn't do it it wouldn't get done.
When I took on my first full time guy I put an ad in the local paper, some national website took it from there and put it on the net (without my permission) as a consequence I got loads of applicants, I was amazed how many were older, many 40+. I really wasn't interested in employing someone over 25 as an apprentice, and TBH I think that many of the older new starts will struggle for several different reasons. I know I'll get a slating for this so I'll say now I have a massive respect for anyone who is willing to start out in this game aged 35+.
These are the problems for older new climbers as I see them.
Physically its just not the same (look at footballers, at 35 its all but over)
Employers want someone to invest in, not someone who will leave and start on their own as soon as they can, as has been said youngsters are far less likely to do this.
Older people in a very physical job are more likely to sustain a lasting cronic injury, you only need to look on here to see the number of people with bad backs, elbows, joints etc, as an employer this is a BIG turnoff, no one wants to pay someone who is off sick all the time or even worse sues due to injury.
Its true that youngsters have less fear, in some cases to the point of recklesness, This lack of fear is bound to be a bonus when learning the job.
Chances are most of these older climbers will have to start on their own as employers will most likely prefer youth, this means they will either have to learn on the job or go contract climber, and as Josh said to get the production necessary to be a contract climber you need to be quick. and that means as quick on friday as you were on monday, not easy as you get older.
I know that there will be climbers out there with 8 years exp like me but aged 45 who could kick my ass in a comp, there will also be those who started young and could beat me.... I'll bet there's a lot more of the latter.
There are probably other pro's and cons but these seem to me the most important as an employer
As a caveat I should mention that I have seen some rubbish youngsters who will never be any good. A good alrounder needs stamina, skill, and balls, but not neccesarily fitness. No good being an ace climber with a silky but brown your pants when the big saw comes out.