Old is a demographic category and as such is a role based on a noun, just as are the roles of mother and soldier. People are old when they are eligible for Social Security, which was set at sixty five when the Social Security Act was passed in 1935 and is moving up from there. Old is also when one has reached retirement age, which used to mean, in the academic community, reaching the age of seventy. Old is more imprecisely defined as when one's body shows the signs of aging to the point that it makes sense to prescribe drugs on the basis of protocols established for old people rather than protocols designed for adults. Age is not just a number but it is that as well. Ask a seventy year old whether he or she would rather be thirty.