I would say that at 31, you are still a baby. I didn't start my full time master's until I was 53 (and my PhD at 55, after a 'gap' year due to funding issues). In my case, making casual friends was easy, though close friends was another matter due to the generation gap. In your case, at 31, I really don't see any issues with the age thing.
As for the intensity of a taught master's that so many people speak of, which in turn would affect the number of hours you can work, don't believe everything you read. A taught master's is what you make of it, and you can read every single item on the reading list plus the weekly required reading, in which case, you'd probably be burnt out even without any part time jobs. On the other hand, you can just choose to focus on the important readings, and those that interest you. If you take this route, you'll probably need to spend 20-30 hours per week on the course. This would include the usual 10-12 contact hours per week, giving you plenty of spare time to do other things, including part time work.
I think most English university split the academic year into three semesters for taught master's courses, with the final semester (June to Sept in my case) being for the dissertation. You should ideally have a good idea of your topic by Christmas.
These are just my experience of doing an English taught Master's in social sciences at a fairly advanced age. Others might have different experiences, especially in different disciplines, but I hope it has given you some ideas of what to expect.