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". The word youth is often used to describe young people between the pre-teen age and early twenties. 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 23 or 25 years old. In many countries, humans become "adults" at age 18 by law, but are still seen as kids/youth by adults. This is partly due to the fact that the mental and emotional development isn't done until around age 25, and also that 18 and 19 year olds are still teenagers, many of them without any adult life experience. Guys and girls will notice a sligtly big difference in their maturity between the age of 18 and age of 25.
For some people, youth is an obsession. They want to look young. Because of this, they use various products like face cream and plastic surgery, to stop or slow their 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.