This is commonly asked in our clinic especially when a young person is noted to have an early cataract.
Cataracts are a degenerative change of the human crystalline lens and occur as part of the normal ageing process. The same advice that your doctor normally gives you, namely a varied and healthy diet full of green leafy vegetables, and colourful fruits full of anti-oxidants, getting plenty of rest and exercise, as well as wearing sunglasses and a hat when outdoors and exposed to the UV rays.
Optimise your health by looking after any other associated medical conditions, and minimise your intake of corticosteroids, and avoid trauma to the eye. You should certainly not smoke, stop smoking and avoid friends who smoke.