I'm a trained singer, and I want to clear up a few misconceptions about the voice getting better with age. There is only one 100% healthy way to sing, and that's without tension. In order to not have any tension in the sound, not only must the singer fully support the breath from the diaphragm, but also have a very loose face and jaw area and lift the soft palette (the roof of your mouth). To imagine a strong breath control take a deep breath while lying on the floor and then very slowly push the air out of your mouth by pushing with your stomach muscles.

The problem with this kind of singing is that, especially in the upper registers, the sound will be operatic and not appropriate for most popular types of music including jazz. A trained singer who delves into these genres must properly balance a little tension with good support to get the sound they want without damaging their voice overtime. It's virtually impossible to not withstand a least some damage to the voice while doing this, however the damage can be minimal with proper knowledge.

Imagine a rubber band pulled out tightly and then plucked. It makes a sound because of vibrations. The vocal cords are the same way only when there's tension in the throat (which is often very necessary for some types of vocal runs) the cords can't vibrate normally. You can imagine overtime this definitely wears them out. Also, the substances we put in our bodies, hydration, and hormones all effect the voice too.

For example (not to be mean here), listen to Demi Lovato's "Skyscraper" if you want to hear tension in the voice. It may add for good effect, but it's not healthy long term. If she sang that song all the time for years, she would loose her singing voice for good.

Mariah has a mixture of tension and well-placed, well-supported moments in her singing (more tension these days due to her shift from diva pop to R&B) which is why she still has a voice to work with although less of one than she did have. Regardless, as a jazz singer myself, I think this song is brilliant. Songs don't necessarily always have to be belted or filled with runs to be good. This to me has many magical moments of harmony between the two, and it works.