If you listen to the amazing voices of Whitney, Mariah, Beyonce, or Brandy, you can hear the distinct progression of their tone, vocal agility, power, as well has their decreasing range. With the gradual change in their range, you can hear more strength, so it actually turns out to be a beneficial asset. The sound is more womanly and broad. Take a listen to Whitney's hit "I Wanna Dance with Somebody," which was released in 1987 when she was 24 years old. Now, Whitney Houston was "The Voice," and she was always a power house singer, but you can hear how her light highs morphed into a somewhat heaver tone by the time she released "I Will Always Love You," just 5 years later at age 29. And of course, if you listen to songs later in her career, like "Celebrate" from the movie Sparkle (2012), you can hear how age shifted her vocal range and tone.
In addition to age, there are a variety of other factors that may alter the voice for women. For instance, pregnancy has the affect of lowering a women's voice by 1.3-2.2 semitones, which is due to hormones. It may take at least a year to get it back, or close, to normal. If you take a listen to Beyonce's break-out song "Crazy in Love," you can hear the young voice, the buoyancy of how she approached notes, and how focused her highs were. Now, if you take a listen to "Daddy Lessons," or "Boss," or even her latest recording of "Spirit" for the Lion King Movie, you can hear how her range and strength has increased drastically. She is now capable of hitting deeper lows than before. Yet, this is over a 13 year span. She has had three children, including a set of twins, since her first single as a solo artist. If you go farther back to when Beyonce was the lead singer of Destiny's Child, you can hear how her voice matured as she developed into a woman. You can also hear the same in Mariah Carey's music and Brandy's music. Although Brandy has always been an alto, she was a teen when she released her first single, "I Wanna Be Down," in 1994. She has fully developed into a sultry alto vocalist.
I must share, how important it is to be mindful of what you consume. Smoking can alter the voice greatly, and shorten its' range. I have seen it first hand in a few of my vocal students. Once, they quit smoking, their range gradually goes back to what should be normal for them. They are able to hit higher notes, and they can sing more songs! Things can affect the voice short term, as well, like alcohol, coffee, dairy, etc. Knowing about vocal health and what your voice needs to perform at its best is education every serious singer should be informed of.
It is possible to expand the vocal range by frequently working at it throughout a long period of time. When I initially meet with a voice student, one of the first things I do is find their range. That way, I can keep tabs on it as I do vocal warm ups, as well as help them stretch it lower or higher. Always do warm ups in small incriments, preferably 2nds, to help ease the range higher or lower. I used to be a 1st soprano, and I worked my range down to a low C. This is probably in part to my advancing age, but I'm sure largely due to my working at it. I have maintained my range for several years now, with my highest note being a C6. This is done by regularly warming up the voice, and practicing songs with broad range and challenges.