|The Simple English Wiktionary has a definition for: old.|
Old is the opposite of new. When something is not considered new anymore, it may be considered old. It is also the opposite of young.
Old can refer to someone or something's age.
Old can mean that something is out of date or past its "sell by" date.
Different things go out of date or grow old at different rates. For example technology has a short life span and can be considered old after only one year. Human beings can by considered young at 20 years old, yet a cat would be considered old at 20 years old, because humans live much longer than cats.