Paragraph by paragraph,
Although it is unlawful, current policy is probably not to recruit any of that age group, this could be chnged
I am sure there are plenty of people available who are suitable, probably if only one in 50 applicants were suitable and fit that would be enough. 15 years out of a staff member would be far better than five out of a younger person who then moves on, which seems to be the issue.
Although many current train drivers do look at retirement once over 50, this does not apply to a large proportion of the population who may not have final salary pensions etc. Also pension benefits have to be earned, joining the job for the benefits and leaving after 1 year would provide pennies of a pension, the only exception being death in service benefit which is not a good reason for taking any job.
Not sure about costs, you would need to review average service periods and also performance of different age groups
Paragraph by paragraph, as you say:
There are currently no solely age-based restrictions on intake (with the exception of not hiring minors). You can't do this, in either direction - you have to be able to demonstrate that an individual candidate is not suitable in some meaningful way. There are a number of older drivers on the network currently, and whilst they're mostly long-serving staff this is more a reflection on the fact that very few people seek a major change in career so late in life. If a 50+-year-old applies and meets the criteria currently, they will be considered based on merit, just the same as any other candidate (though sadly due to the exceptionally high standards from a medical perspective, many may be ruled out).
Arguably a 50-year-old who is later on in life and may not require the same job security and other aspects as, for example, a younger person with a family to support, and therefore may actually represent more of a flight risk. Age is not a good indicator as to the potential duration of an employees service. (I'm speaking from experience as a hiring manager and department head in several companies here.)
Whilst purely monetary pension benefits have to be earned, you have to remember that jobs in the rail industry often come with other benefits beyond an employer-provided pension that will transition with the employee into retirement as part of their pension "package" - such as free/discount travel, medical cover, etc. Given that these are also high salary jobs, and that any pension contributions are taxed at the point of draw-down not earning, it's very possible for the pension benefits offered in such a role to have a very meaningful impact on the employee's retirement income over such a short period. Think voluntary employee contributions (perhaps with matched employer contributions), the effect on tax bands, etc.
As ModernRailways says, the solution to this is to fix the actual problem and address the reasons staff leave so quickly. You can't fix these kinds of retention issues by putting weird policies in place to recruit from certain pools of workers who will tolerate poor conditions and management nonsense better than others - the issue will always keep coming back.