Hi, White hair in beard at the age of 24 years is quite unusual. This is known as premature canities. The latest of the treatment available for the same is by Melitain gel local application. For the prevention there is no mainstay of therapy but you can take vitamins in the form of biotin, calicium pantothenate and other B complexes. Eat a lot of green leafy vegetables and sprouts which contains vitamins and minerals necessary for normal hair function. Reduce your stress level and stop any substance abuse like smoking or alcohol. Take care.