Age

From Peter's section, "I am not old, he cried. . . He was not old, or set, or dried in the least" (Woolf 50), I began to think about age and old versus young. Age is defined by google as, "the length of time that a person has lived or a thing has existed". 
Young is defined as, "having lived or existed for only a short time" and old is defined as, "having lived for a long time; no longer young". 
But I think that age is so much more than just young and old. Like the poem we read in class, every year you become all of the years that you have already been. Like if you're turning 5 then you're also 4, 3, 2, and 1. 
At what year does someone become old? Can someone who is 80 still be considered young? I think that young versus old has to do with your spirit and the way that you live. If you're full of life and still able to live life to the fullest, I believe that you would be considered young. If you can't do that, I believe you're considered old. 
But even then, what about people who are depressed and not full of life because of their illness? How are they considered old or young? Does that have to do with a specific age? 
I believe a person should be able to decide whether they're young or old. It becomes a self-reflection for them, as they have to consider what makes them young or old. 
For me, I believe I am young because I'm not yet 18, which is considered a legal adult. But who made this rule, that at 18 someone becomes a legal adult? Whose call is it to make, as some 18 year olds are so much mature than others? And when I'm 18, will I be considered old because I'm a legal adult? 
Maturity also plays into age; if you're immature and juvenile you're considered to be young, and if you're mature you're considered to be older. 
When you think of old, you immediately think of a grandparent and when you think of young, you immediately think of a child. But there's so much in between, say from 18-70. What are these people considered? 
I believe age and the idea of young and old has stemmed from society and social norms. I honestly don't think that any of this matters or that even these words should exist in our society because everyone is so different, it's hard to classify and generalize age and youth.