Your vocal chords are muscles: if you don't work them out they will get saggy and loose, which leads to crappy singing. Does that make the voice tonality change? Not much, I think. I know that really old people (50+) say that their range tends to go lower: they can now sing lower than the used to be able to, but not as high. But just listen to singers or even "vocalists" who aren't even trained of bands that have been together for a few decades: they still sound like they did when they were 21. Just my observations. :D

Most women's voice settle by about 18-20. Most men's voices don't truly settle until around 25 or so.
Unless used improperly, range should stay pretty constant although men tend to lose their "falsetto" range if its not used. A properly produced head voice should stay with a trained singer throughout their life.
With training, singers can sing wonderfully until their 70's/80s. Often a darker color starts to appear but range is rarely affected unless outside influences (misuse, alcohol, smoking, etc...) are involved.

BTW- the vocal folds (chords, bands) are not really muscles, they are however controlled by muscles and tonal quality of a voice is greatly affected by the muscles in the neck, face and throat.
Most of a singer's tone is dependent on physical make-up and "concept" of tone, that is, what tone they are trying to produce.
A good vocal coach can truly help with tone production.

Interesting! I guess what I am looking for then is the tone of voice. I really should get some singing lessons. Thanks for the responses!

Some male voices get deeper and lower, while others get thinner and higher as they age. (I've heard arguments that's a testosterone thing, but I'm not qualified to speculate there.)

One voice that you can probably hear this change happen to is John Fogerty. Listen to his early gruff-growly sound with CCR in the early days; late 60s. (He's been compared to the vocal equivalent of gargling with Drano.) He was a full time smoker as well.

Listen to any of his studio recordings as a solo artist since "Center Field" (early 80's) and you'll hear a distinct change in his voice. He stopped smoking, and as he's aged, his voice sounds a bit thinner, and even higher. Of course, artistic temperament has a lot to do with it as well, and he's no longer trying to burn down the stage with his voice alone, but STILL, you can hear a major difference in the timbre of his voice. His vocal sound on his latest CD "Revival" is a world away from the days of "Born on the Bayou" and "Chooglin" with CCR in the 60s.

There's plenty more examples out there, but he's one of those that I've paid attention to over the years.

Take care of your voice as you go along; see a throat specialist from time to time, and you'll have years of great singing ahead of you.

pmolsonmus is quite correct- your vox is still growing up to mid wtenties.

btw:
when i was your age the movie "The Committments" came out. The singer in that film had a great bluesey raspy ballsy sound that I so wanted. So I started smoking.

I am now 37, a pack a day smoker for 20 years and I still sound nothing like that guy!!

You forgot the whiskey ingredient :-)