You won't get sick immediately from eating old cooking oil, but having oil that has been sitting around for a while means the oil has very likely gone rancid.

Washing the hair with soap removes less oil from the hair, washing the hair with shampoo removes more oil.

The main important point is not to shampoo your hair too often. Depending on the culture in which you live, washing the hair no more often than every 1-4 weeks may be the best way to have healthy hair.