I would probably prefer to hire a sharp high school kid over a 30 year old. Most of the older folks who I saw get hired struggled through most of their careers, especially at the busier facilities. I like to tell people that the ideal applicant for a controller job would be a young kid who gets a lot of speeding tickets but doesn't get into accidents! On the other hand, it's a fact of life that our skills deteriorate when we get older. The age 56 rule (just like the age 60/65 rule for pilots) exists because too many folks are in denial.
As far as the original topic is concerned, I never really agreed with the practice of combining the positions in order to bury the trainee. This usually just destroys the confidence of the trainee, creates a safety hazard and delays, and wastes everyone's time. I would rather see the trainee master the basics, so that they would know their limits and have something to fall back on when the chips were down.