If it's a fixie, that means no freewheel. Fixed gear means fixed gear, you can't coast.
I do have a fixie, a Kona Paddy Wagon. However, it has a flip-flop hub - fixed on one side, freewheel on the other, so by turning the rear wheel around one can choose to ride fixed or singlespeed.
I ride 42-16, 70 gear inches. It's a good compromise between speed on the flats and being able to get over the climbs. I can struggle up anything up to abput 10%. Steeper than that and it had better be very short.
As for your age, I'm in my late fifties, this is my first fixie, I bought it when I was 56. It takes a little getting used to but it is a lot of fun, and it has become my go-to bike for just riding around. Plus it makes every hill a strength workout and promotes smoothness in the pedal stroke as you learn to spin downhill at what most people regard as ridiculous cadences.
Whether you should convert it to a fixie depends on taste. Restoring it to its former glory as a 10-speed might make more sense for you. But don't be put off a fixie because of your age, that's really immaterial.