youth is based on many factrs. 1 genetics 2 how you physically look 3 how you act/ think 4 how others see you 5 your actual age. (which is last coz its man made)
Interestingly I have not posed this questions just for a bit of fun - it is a research topic that I am increasingly interested in and there can be many perceptions of what youth is I need to know what yours is. Those that can not be bothered with answering simply 'dont answer!
Your sarcasm does not wash with me and highlights the fact that the people within this arena are lacking the intellegence to delve a little deeper than the front page of 'the sun' - dont worry ill take my daft questions elsewhere in order to get some substantial and though provoking answers, something you are obviously not capable of doctor spock.
kdm, I read your qeustion as 'a youth' apologies for that.
Youth itself I would say is the time in someones life which occurs between childhood and adulthood (suggesting those paramters aforementioned). Although there are some who say:
"Youth... those persons falling between the ages of 15 and 24 years inclusive." - United Nations General Assembly
"A person... under 21 years of age." - US Government
As mentioned above, an Octogenarian can have youthful feelings (state of mind), but won't be classed as youth.