Apart from the social understanding that quantifies the young category under a time frame, I think being young is about having a tender heart, a heart that is able to smile and be surprised and rejoice and hope and be enthusiastic about life and that is joyful in living for the glory of God. Remember that Jesus said: "Let the little children come to me and do not hinder them, for to such belongs the kingdom of heaven", so it is this youth that counts, not the time you have on earth, but the tender and sweet heart that God works in you.