Apple turns down an actual genius for Genius Bar job

Opps but I guess you're stuffed once that crystal in your hand changes colour as far as a job in an Apple Store is concerned.
Having said that, people who are technical geniuses aren't necessarily also good at dealing directly with the public.

Perhaps a little over qualified to do what is in fact first line support?

I suspect this is what happened; hope it isn't ageism.

There may be an issue not with his age being anything to do with his ability to do the job, but the perception of his ability to do the job. Put it this way, honestly now, if you go into an Apple shop with say a problem with an iPhone and the 'Genius' you get pointed to is in his 50's, quite a lot of people ARE immediately going to think 'what the heck does a dude that age know about smartphones?'
You'd be wrong of course - although to be fair figuring out how to port an OS doesn't automatically mean you're good at solving 'user space' problems, although I'd expect the selection process to include aptitude testing of some sort - but that doesn't change the fact that that would often be people's first impression. And the Apple Shops are run by sales people, who generally think first impressions are pretty much the most important part.

Nevertheless, it does highlight an issue we have and are going to have more - we're going to have more older people looking for jobs as retirement ages rise and job security weakens. There are going to be more and more people in their 50's and 60's doing jobs we traditionally associate with people who are at the start of their careers.

I think that's a complicated question to answer and would very much also depend on the age of the customer as well.

If the customer is relatively young they may indeed wonder what the heck an old codger is going to be able to do to fix their smartphone or computer. On the other hand, a customer in their 40's, 50's or 60's may well prefer to deal with someone who looks like they've got a bit of experience and is perhaps a bit closer to their own age and who they don't think will treat them as an imbecile just because they are a bit older. I wouldn't necessarily place any more confidence in someone who looks like they've just come out of secondary school being behind the genius bar than I would in someone who looks like they'd be happier down an allotment or watching Countdown.
Case in point, if there's a problem with my PC at work I have rather more confidence it'll be fixed when the older support guy picks up the ticket than I am when it gets picked up by the guy who looks like he's about 12 (although who is in fact in his early 20's).

It depends, I'd probably be more comfortable with a 40+ dealing with my problems than a <25...

Having worked at a software developer, where I was the only programmer over 30 (in fact I was 40), I was also the only one with the ability to tune their database queries and optimize their code, so that it could cope with large numbers of visitors. Nobody had taught them (at school or uni) how to write optimized code, it was all written to be human readable and "pretty".

That meant, when the server was under load, it would take around 1 minute to load the main menu for the eShop. The DB admin had to come in every time high load was expected (E.g. PayPal newsletter, new collection etc.). That was with 4 load balanced servers and 250 simultaneous visitors. I managed to optimize the code after a few hours, so that the main page loaded in a in under a quarter of a second! The next time the store appeared in the PayPal newsletter, the servers could cope with over 200 users each and generate the menu in under 1 second.

All through looking at the database design and reordering the query to be optimal for the RDBMS and not optimal for a human to understand it. The same with the code itself. Much of it was written to be logical for a human reader. But nobody in the company had ever heard of optimizing for the processor. Changing some IFs from being logical (If NOT a then b else c) to be machine optimized (if a then c else b). For a human, the first is more logical to read, but wastes a lot of cycles, because 90% of the time a is true and it is cheaper to test a positive and it is cheaper to not have to skip over the negative code and execute the fail code. For 1 user, it probably won't make much difference for users of modern PC. But use that code with hundreds or thousands of requests per second and it makes a big difference if you are wasting a few hundred clock cycles per request.

I have had similar experiences with young sales and support staff in shops and companies. They might be friendly, but if you ask them a question that deviates from their script, they often can't adapt and give a sensible answer.

Maybe they realised how lonely he'd be there.

Effing hell, have you ever actually BEEN in an Apple shop?

I said lonely, not alone. FFS.