The voice mechanism is a delicate, yet strong instrument, and can last a lifetime; provided it is not abused.
Voices however, are under more stress now than they ever used to be; orchestras are larger, the diapason is higher, because of air travel singers are more tempted to take on too much work coupled with shorter rehearsal times, and jumping from one role to another without sufficient recovery time.
Vocal repair is therefore dependant on the individual, and what fits one singer does not necessarily fit another. It can be very difficult for a singer of a certain age to believe that there is an answer to their vocal problems especially if the singer is/was well known. But they do not stop to seek a solution and continue their career regardless, or, they often seek an answer and find nothing, both resulting in more vocal decline.
It is very important for the singer to realise which muscles are doing the work, for example; many singers inadvertently use the swallowing muscles to form vowels and tighten the root of the tongue. This causes a loss of range as the larynx is restricted in its movement. Also, the space inside the throat is lessened. It does not matter what the vowel is, as the yawning muscles should dominate, allowing the throat to stay open. The vowels are made by the ear and not the throat muscles. I am fully aware that this idea may seem strange and impossible, similar to a ventriloquist! You do not have to do anything physically. It is exactly the same concentration of the right ear where you physically sing the note, you do not have to add anything extra!
The breath similarly must be free of tension. The old Bel Canto singers were trained by using a lighted candle and if the flame wavered, it represented a waste of breath and poor tone. Therefore, instead of thinking the voice as coming forward out of your mouth, imagine it travelling backwards. The strange irony is that it feels like an inhalation, rather than an exhalation.
It does not matter whether the pitch is going up or down, the same sensation is there all the time. It is the intensity which changes. The vocal cords are responding to the ear and the nervous system and adjust their length automatically via the arytenoid cartilages. This is truly miraculous! It also means that the saliva glands lubricate the vocal cords without hindrance, and the mouth can be used economically.
It is interesting to note that the ears are sited in line with the back of the throat, away from the lips!