Keep your voice young. It’s much easier than keeping the rest of your body young. That goes for your speaking voice as well as your singing voice.
As a veteran chorister, I had thought I would have to retire at a certain point – until I heard Charles Aznavour singing at 91 and Tony Bennett at 88. Their range is smaller and they’ve both lost a bit of smoothness, but each can still sing for 1-2 hours with a voice barely changed from when they were young.
Nothing is free after 60. I spoke to a classmate at a high school reunion a few years ago, and he could barely speak halfway through the evening. I asked him if he had a cold, and he said, “No. I just never talk anymore.” He lived alone and apparently did not even talk to himself (I would be mumbling all the time). He then moved into a senior living arrangement and at the recent reunion, his voice was fine. He’s been talking.
Here are a few simple exercises you can do to keep your voice supple.
Hum very lightly on a single pitch for 10 seconds, change the pitch, hum 10 seconds, and so on, going up and down your range. It is surprisingly challenging in the beginning, but you’ll get it. When you have mastered 10 seconds, move to 15 seconds. I do this exercise in the shower, while driving, in the elevator…anywhere.
The ujayi breathing common to yoga is also a big help in keeping the voice young, though this takes more attention than the 10 second humming. With the mouth closed, you draw breath in to the back of your throat, then breathe out the same way, creating a deep, open breath.
If you can make a horse face, going brrrrrrrrrrr with the lips flapping against each other loosely, that will also help the voice. You can do that without humming, or humming on pitch. To relax the lips more easily, bend over.
Thanks to @Francesmarsh for recommending @voicelessons.com, where you can find a hundred or more short vocal exercises.