I could talk about this all day.
I'm an MCCi and TKI and instruct at several ATOs.
The students I see have an incredibly wide range of skills, knowledge and attitudes. Literally incredible, as in I have difficulty believing that some of these people have ever sat in an aircraft (no scan, no R/T, multiple basic handling errors, no knowledge) while at the other end of the scale some of them you'd think already had 6 months on the line.
This suggests systemic problems in training and examination standards.
Yes, it's anecdotal, but based on the cohort of students I've seen (trained at many different ATOs) then if it was down to me to dole out jobs at airlines the breakdown is something like this:
10% just terrible. No chance of passing a selection or a type rating course, and fundamentally untrainable. (how did this person pass an IR?). Recommendation: Give up.
40% to varying degrees with serious gaps in capability. Far too much of a training risk to be given a job (if my neck was on the block over it) BUT trainable. With proper remedial training could be brought up to a reasonable standard given enough time.
35% pretty reasonable. Trainable albeit with a few weaknesses. Slight risk, but ought to be ok on a type rating with decent training support.
10% Good. Shouldn't be any training risk at all.
5% Excellent. Forget the type rating, I'd fly the line with this person right now.
Age is much less of a factor than previous quality of training.