55 years old here. Got my first single speed since I was a kid last summer. Sure it isn't a MTB or a fixie/track bike, but a diamond frame adult sized city cruiser with 700x42 tires and 66 gear inches. The only hills here are the wind or man made. This machine sees mostly pavement, but I take it down dirt and gravel paths and tracks as it does so easily.
The single speed is simple and doesn't drop the chain. It makes me decide to accelerate slowly or stand up to accelerate quickly. In fact this single speed bike makes me stand up much more than a geared bike as I can't merely downshift for lower gearing. It turns out that getting out of the saddle works me out better than being glued to the saddle, which also makes my bum last longer on a ride.
My only geared bike at this time has a 8 speed IGH with as high as 22% between gears. Riding the single speed has effectively made those wide gear steps on the 8 speed feel much closer together.
For whatever reason, a single speed bike makes me feel younger mentally and physically. It's just a different kind of fun. I like it.
