I too believe the fact that life begins for us when we are born and once we are born we move ahead in our lives and cross different stages of life during our life time...
LIFE BEGINS AT THE YOUNG AGE THAT IS FROM 25 OF AGE.
WE CAN EVEN SAY THAT WHEN WE KNOW OUR RESPONSIBILITY AND RESPECT THEM AND FOLLOW THEM. THEN LIFE BEGINS.
I think as of now that there is no hard and fast rule about this and life begins whenever you want to make a new and positive beginning irrespective of age...!