Many mountain bikers are exhibiting what may be called "single-speed syndrome." While these may be purists in the sense of keeping nature's exercise as simple as possible, single-speed bicycles promise to be a good way to get outside for some exercise, fresh air, and fun.
However for the average baby boomer, and I think for most boomers, riding with a single-speed bicycle would be done on more level bike paths and probably city parks.
There's a reason the first mountain bikes evolved into the multi-gear, full-suspension beasts they are now. Pedaling a stiff single speed is just plain hard work. I can't imagine going mountain biking with just one gear. I can certainly admire people who are physically capable enough to be able to climb those hills without gear. To buy the best bike, you may check Singlespeed shop near me from the internet.
A while back, I sold my mountain bike and bought a new hybrid bike so I could ride more on the road and enjoy fewer off-road trails. Make sure any bicycle you decide on suits you, your needs, and your lifestyle.
Single-speed bicycles offer a simple, somewhat retro way to go outside and enjoy the nice summer days. How much you want to spend on a new bike depends on how, where, and how much you're going to ride. Most good local bike shops will have experienced and well-trained staff to offer advice and suggestions on what would be the best bike for you.