I think the age barrier is caused by the fact that airline pilots retire at 55 years of age. For sponsorship (whether full, partial, or simply a 737 rating for a new ATPL'er), an airline has to pay x thousands and within <two years, they have a pilot who they will theoretically keep until they are 55.
A 28 year old cadet will become a FO at the age of 30 and will have 25 years potential service.
A 20 year old will become a FO at the age of 22 and will have 33 years potential service, thereby delivering a greater return for the original outlay.
Can someone clarify how easy it is to get into a Cargo Airline now at a late age ? A Cargo Airline won't benefit from the above equation as a young pilot is inevitably going to ditch them for a higher paid airline job at the earliest opportunity. An older person doesn't have the luxury of doing this and hence would be more likely to serve more years service in a Cargo Carrier.