The traditional point of view in classical vocal pedagogy was that you could not start training the voice until after puberty and that after about 30 your voice was set and could not be changed.
We know now this is false. It is possible to train a child to sing well (as long as you train the child to sound and behave like a child and not a mini-adult). It is also very possible to train an adult of any age, even a “senior citizen”, and get very good results, provided they understand you, are healthy, and practice. Over the decades I have worked with many older singers and in every case, with the above criteria in place, seen them improve in their singing. Willingness, persistence and a positive attitude are the most important ingredients. In fact, younger people with less life experience can be much harder to teach, in that they can have a limited understanding of process-oriented work. Learning to sing well takes time and older people know that many things in life work that way.
Obviously, someone who has singing in their background has an advantage, even if was decades ago that they had sung. The muscle memory is there, somewhere, and also the mental awareness, and that can help when “restarting” the engine. Someone who has never sung, however, but who is highly motivated can make progress if they are consistent in their studies. In fact, even people who have been told repeatedly that they “can’t sing” or are “hopeless” have done well with me, slowly, and over time. Why? Why would it be that they make progress with me when they have been written off by another teacher, sometime several other teachers of singing.
This can partly be blamed on the lack of underlying structure within the profession that would allow a broad dissemination of information that could benefit everyone. If someone has the capacity to teach people who are not at all able to sing, to sing, there must be something going on there that was not available to the other teachers. What that something is ought to be of interest to virtually everyone involved in singing. Sadly, that’s not the case. People who don’t know that they don’t know (and there are many) are often suspicious of information they have never seen. Crazy, cyclical thinking, I know, but a real circumstance.
So, if you have been told that older people can’t learn to sing, or if you believe that you can’t start singing training until after puberty, there are new things to learn that should change your mind. If you think that there is just one way to teach someone to sing (your way) and that you can’t find a way to work with someone who is motivated, sane, and willing such that they learn to sing and enjoy singing at almost any age, there are new things to learn that should change your mind. Look around. Read. Talk to others. Open up your world. Age is not a factor when teaching singing or learning to sing.