I would say that age isn't necessarily an issue, but the level of maturity and the life skills that come with age are. As mentioned previously, the majority of those I trained with on an integrated course were under 20 years old, had no further education and or never had a job. On the flight deck, the young and old tended to perform the same. Results were similar, and flying performance was dependant on ability, rather than age. So flight school performance was pretty much the same.
However, there was a world of difference when it came to what went on outside of the cockpit. The slightly older ones could stand on their own two feet, make their own decisions, and tackle their own problems.
The younger bunch were much more irresponsible, made rash decisions, and you could say were a lot more socially awkward. A lot had quite passive personalities, and only seemed to speak up when they had no other option. An older student would knock on the CFI's door and talk face-to-face about a problem. The younger ones would e-mail them, or get their parents to do it. Jokes aside, there were integrated students who used to get Mum and Dad involved when things weren't going their way. The younger ones seemed to have much less real-world knowledge, and had unrealistic expectations about what was going to happen. For example, thinking they'd be hired by British Airways or Etihad straight from flight school. Just generally lacking the life-skills and view on reality you get from the real world of work. This is a generalisation, but I assure you that the bulk of integrated schools are full of students like this.
From my experience, I can partly comprehend that 50% of integrated graduates aren't suited to airline work. Mainly due to the high proportion of very young and green graduate pilots, without the life experience gained from ever having a job or further education. Lack of maturity is definitely an issue.
But, as of late I've heard that the youngest integrated graduates (19 or 20 years old) are having much more trouble finding jobs. And around two years ago I was told by a recruiter that an airline job becomes extremely hard to find if you're over 30 years old. So in my experience I would agree that it's a young man's game at the moment, with mid-20s being the peak age.