Among singing teachers, there is a general consensus that heavyweight singing - big operatic or music theatre roles, for example - is bad for young voices. In general I would agree: overstressing voices that are not fully developed tires them and can wear them out before their prime. But there are exceptions.
I am currently teaching a 17-year old soprano. She's not been studying with me very long, but has had singing lessons with another teacher for several years. When she started with me she was singing light soprano repertoire, which would normally be the correct choice for a girl of her age. Her voice was attractively light and bright, but her top notes were restricted and her voice was breathy in the middle and lacking "shine". Her breathing was too high and shallow and there was tension in her shoulders. So I encouraged her to breathe more deeply, relax her shoulders and support her voice from deeper in her body. She did this to some extent in exercises, and what began to appear was a much bigger voice with a real lyric warmth and some of the "shine" that I longed to hear. The trouble was that as soon as she started to sing the light soprano repertoire it all disappeared and we were back to shoulder tension, shallow breathing and breathiness.
Today I gave her Brahms "Immer Leiser" in the high key to try. This is a thoroughly miserable song that requires dark colours and a big lyric sound to work well - completely the opposite of what she had been doing, and not everyone's medicine for a young voice. I encouraged her to make the German vowels as dark as possible - so that her voice sounded "too dark" to her - and to "sock out" the high notes rather than trying to "place and float" them as she had been taught. Singing like this, she sang her way through the piece with complete freedom and produced at the end the easiest top A she has ever sung. And she made the most glorious lyric soprano sound. Wonderful.
At the end of the lesson she said, "That was so easy! And it's the first time my throat hasn't felt sore at the end of a lesson!". I was horrified. Worried that I had been doing something wrong, I asked her if her throat had just been getting sore in the lessons she had had with me. "No," she said, "it's every lesson I've ever had, and rehearsals and concerts as well. I have to have "quiet days" after singing. I thought it was normal!"
Readers of my website will know that Coppola Rule No.1 is "If it hurts, STOP!" Throat soreness after singing is ALWAYS a sign that something is wrong. This girl had been singing in too tight and gripped a way for years - all of it in the name of not overstressing a young voice. The repertoire she has been singing is too light for her and she has never been able to let out that lovely sound - in fact she has been led to believe that singing with her natural rich, warm, vibrant sound is wrong. Even at 17, she needs to be singing bigger things.
So light is not always best for young voices. It depends on the voice. It is the teacher's job to discern what the voice needs in order to develop and set appropriate repertoire. And if that means giving an 18-year-old Madame Butterfly to sing, then do it - and damn the rules.