Can a student be too old or too young to benefit from studying voice?
First of all, a student can never be too old to improve their voice through study. Just as with any age, you can learn to strengthen and control the muscles involved in singing. If you are singing with good technique, your voice will only improve as you get older. As far as younger students go, I suggest that students should be old enough to maintain focus through a half hour vocal lesson, and be able to practice at least 10-20 minutes per day.