The ability of the lens to change its shape and therefore focal length (accommodation) depends on its elasticity and normally this is gradually lost with age. After the age of 50 the lens is usually unable to acommodate fully and a form of long-sightedness known as Presbyopia occurs. The effects start with difficulty in reading small print in poor light. This condition makes it harder to see close up, but you can see things far away just fine. The condition normally requires a minor correction with a weak convex lens.