There is no age limit to wear glasses. You shall take your daughter to eye doctor. If s/he suggest that your daughter can wear glasses/ sunglasses, you can buy a pair for her according to their vision requirements. If the doctor not suggest you to buy glasses, you shall not let your daughter wear glasses for the sake of her eye health.
Generally children older than 3 years old can wear sunglasses. Exposure to ultraviolet rays may raise her risk for cataracts and other eye problems later in life. But putting bad quality sunglasses on your child is worse than his wearing no glasses at all. So make sure you choose good sunglasses offering 100% UV protection for him.
It is said that children should start to wear sunglasses from the age of three. Childrens' eyes are easier to get damage from the the sun than adults' eyes. Because childrens' eyes allow more light to reach the retina. Too much exposure to the UV rays can make them at risk of cataracts, pinguecula and pterygium. When you choose sunglasses for your child, be sure to choose one that offers 100% UV protection and they should be made of durable materials.