There are conditions like severe dry eyes, treatment with Isotretinoin for acne, congenital corneal anomalies, glaucoma, cataract, pathological myopia/ hypermetropia, keratoconus, and very high powers, which are considered unsafe. The pre-operative evaluation for refractive surgery will help identify if you have any abnormalities that may exclude you from having surgery but being 40 or above is not one of them.