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People fail because it is a difficult, intense course. The age profile has a lot to do with it. In the 1970s when I did my course, I think the maximum age for the long term course was 25. For the short term course for those with a relevant background it was 35. My long term course had twenty on it and all passed.
Now days employers are not permitted to discriminate on age. Some courses have had trainees in their mid to high 40s. The pass rate for trainees in their 20s is around 80% whereas for those in their 40s it drops to 20% (roughly).