A lack of water is unlikely to have any major effect on hair growth and quality; at the most, regular dehydration may cause hair to look a bit dull and limp. A lack of water will affect your health and body functions in general, so your hair may in turn be affected, but it is unlikely to be solely or even partly the cause of hair loss.
Hair growth is a complex process that is affected by very many factors, and it is difficult to determine exactly which factors are causing a decline in hair growth and quality without professional help. However, before visiting a hair specialist, you should try to note changes in your lifestyle over the last 10 years or so that could affect your hair growth. Stress usually increases as we grow older and get more responsibilities, and if you have a tough, stressful job, you will in addition probably be sleeping less, eating irregularly and unhealthily, and tiring yourself out. Ensure that you are directly taking good care of your hair too - wash it regularly using a shampoo that is suitable for you, if necessary use a conditioner, oil your hair at least a couple of times a week, and in general avoid being rough and hasty with it and minimize your use of harsh cosmetics such as dyes and gels.
answered by M W