I am 73. I got a 2:1 in 1969 from a Russell Group University. I want to do a Masters in Philosophy at Edinburgh. I think I will start with the PG Certificate (only 60 credits needed). I am wondering whether I am taking on too much and/or whether they will consider me.

I read of several instances where people older than you have successful completed a university degree. I would write a cover letter explaining the motivation and the potential to pursue a Master degree.
I wish you good luck.

They will definitely consider you.

Whilst you've been away from academia, it has shifted a bit towards a buyers' market for masters. It is still not simply a case of having the cash to get into a good uni (such as Edinburgh), but it is perhaps less meritocratic than it used to be (but still, certainly, just as nepotic).

As a result, do investigate the course when applying. It might not be so much that you're passionate about career opportunities afterwards, but you will likely want good quality lectures, discussion, and support from the academic staff. Academia has grown exponentially, and this means whilst it used to be somewhat a guarantee as a student you'd be taught by a lecturer knowledgeable in the subject, it may now be the case that a cover or less-qualified staff member handles a masters as they, particularly 'conversion' masters (where little or no a priori knowledge is assumed), are sometimes cynically viewed as high-turnover, highly-profitability operations.

Also investigate the 'staged' approach carefully. It may be helpful to you; it may also cost more. It's sad when we're in a day and age when you have to consider if a University is basically attempting to fleece you for as much cash as it can get, but it's sadly sometimes the case.

It's hard without knowing you to understand if you're taking on too much. As a middle-aged academic I've had several PhD students much older than myself and whilst they might not have had great methodological or exam-based know-how, they did have much more wisdom and life experience than me and it really was a mutually beneficial experience. There is a great benefit in people with a lifetime of knowledge wanting to apply the rigour and method of academia to translate this knowledge into something objective, and this can be immensely rewarding individually and societally. Academia does not go out of it's way as much as it should to facilitate this, but it also (in my experience) is open to it and does not close doors.