A number of extrinsic or external factors can act individually or together on the normal ageing process to age our skin prematurely. Most premature skin ageing is caused by sun exposure. Other external factors that prematurely age our skin are repetitive facial expressions, gravity, sleeping positions, diet and smoking.
In conclusion, ageing is a complex process involving intrinsic factors (genetics) and extrinsic factors (especially sun exposure). Ageing is not a disease but a normal process that happens to everyone.