Its more to do with the person and their attitude than age...
my best guys are in their late twenties ( started in their early twenties ) and been great - they clean more and longer hours than i ever could do. just taken on an apprentice who has a great attitude to work. and i have a chap who's now 61, never misses a day, never late, never sick.
we have had to throw some back into the great unwashed both oldies and noobies in life rich tapestry, usually know within 4 weeks if they are a keeper or not
look luck with him Matt
Darran