Logic won't work in this case
The usual arguments against older low experienced pilots like adaptability, trainability, health and wot not are tedious and misguided.
Older inexperienced pilots have no problems learning new stuff either. Anybody who managed to pass ATPL exams without too much bother and got his CPL ME/IR without too much difficulty (partial pass or 2nd attempt) is unlikely to struggle with any further training. Anybody who is happy to upsticks and travel the world to get that job has no issues with adaptability or flexibility and to suggest otherwise without further assessment (i.e. an interview) is a tad irrational.
Of course, sometimes the CP just can't get his head around older trainees, because it wasn't like that in his day. But I think also that many restrictions actually stem from the national authority. Stuff, like the need to have an ATPL or 500 hours on type etc. These are usually requirements for transfer licenses and ratings. The ability for an airline to get approval for cadets (low experienced guys of whatever age) is often quite long winded (months of work) and tedious. Most don't bother. Sometimes it is easier to get approval buy narrowing the scope to fit the usual and naive received wisdoms.
I have never been able to deduce a clear rationale.