I think the 60 age is because traditionally women retired then, and men couldn’t be disadvantaged by still having to pay, but I think 60 has been retained because many people begin to develop health problems then, and so they can still get their medicines free of charge.
I’m sure some bright spark will decide to change the policy at some point, but it remains unchanged at the moment.
When I was working, I bought the yearly prepayment certificate as I suffer from conditions that need medication permanently. Asthma is one of them and I have three different medications for that alone and have done for many years. Additionally I sometimes take steroids for it too. And that’s just one condition. The cost would be considerable if each item was paid for separately, which is why I got the prepayment certificate.