What causes the voice change as our parent ages?

Medications can change voices--most notably, hormones, but many others.
Mom has medical conditions that can impair voice/breathing, such as obesity and heart issues physically impairing breath, as well as energy and muscle use needed to speak.
If there has been any type of neurological impairment or stroke, it can change a voice.
When there is voice impairment, check if there is a bit of deficit in swallowing ability.
Might also want to check if there might be a polyp or other growth on the vocal cords.
A Doc who knee-jerk answers "aging" when there is sudden onset to any condition, is out of line--unless they do not care if there is something wrong at that age!
Sudden onset, or else, being able to point to a certain fixed time that a condition started happening, means something went haywire that ought to be checked into.
"Aging" creeps up slowly over time, so that most often, one cannot point to a specific start time for it.

The above answers are true and correct. There's a possibility it could be something else and correctable. Never hurts to check. A speech/language pathologist couldfind problems if they are there and there are ways to correct many of them.

Its so funny that you should ask that question. I just took Dad to the doctor for a follow up on something unrelated to voice. He had throat cancer back in 1980 and one of the symptoms was hoarseness. He's not hoarse right now, but his voice is so weak and high pitched. So, we asked the doctor why Dad's voice is the way it is. The doctor looked in my dad's throat and said he didn't see anything to be alarmed about. Then, he asked my dad how old he is. Dad said 86 (well, actually, he looked confused like he didn't know how old he is, or didn't hear, so Mom answered for him). The doctor said, well, your voice is 86 years old too! Made us laugh. Thanks, JaneB, for giving the real reason!

As muscles get weaker, there is less air to produce the voice...and less resiliency in all the muscles that create sound. The vocal cords are really not cords, but are attached to muscles themselves. As they vibrate differently, they create a different sound. Nothing is wrong....it's just what happens.