Youth is the opposite of elderly. When something is new, it can be called young. Sometimes "young" is only for living things, and things that are not alive are"new". Old can refer to someone or something's age. Young can mean that something is current to date or well before its "best by" date. Different things lose their youth or grow old at different rates. For example, technology has a short life span and can only be considered young for roughly one year. In many countries, humans are no longer young at 18 or 20 years old.
For some people, youth is an obsession. They want to look young. Because of this, many anti-ageing products like face cream and plastic surgery, have been used to try and stop people's skin from looking older. The "Fountain of Youth" was a popular myth, and people thought that if you took water from the fountain, you would never be old. Now however, we know that this was a myth, and it was not magical at all.