"Whaddaya Mean You Want Some ID?"

I have three situations that are my best thinking times.  One of those situations is resting on my bed.  Not really relevant but if you want a mental image of me thinking, there it is.  So I was doing what I do when I’m bored, writing stories that I have no plan on sharing to the world.  Because I don’t plan to share the full story, I’ll share the relevant parts for the sake of this post.  Orphan girl kills a boy’s parents because she has her sadistic side, so afterwards they have to find a way to coexist in the hidden-from-the-law world they live in since there is nobody else to help them but themselves.  A pretty uneasy alliance if anything.  So in my head I always thought to myself that the girl was like a year younger than the boy, but then while thinking on my bed, I realized “How in the world would I know that?”  

In my mind, she wouldn’t have known her birthday or birthdate, she is a street orphan, so how am I to say how old she is?  If I believe it, then the other characters would unconsciously believe the same since I’m the writer and decide how they think.  For some reason, this thought really threw me.  

If I didn’t know someone’s birthday or anything about them (such as year in high school which would give away age), could I still guess how old they are?  I really doubt it.  I wouldn’t assume Aarti was 15 by looking at her since there are girls smaller than her but older.  I felt it was weird that there is this number that holds so much weight in society, but has no practical use.  

So many things get decided based on age.  From driver’s license to year in school to legal limits for certain…activities.   In addition, Minors get punished differently than adults…I could keep going.  If that is how society judges age, what about without society?  Age doesn’t signify growth; I can’t judge people’s ages based on their height and weight.  Age doesn’t signify maturity (I am a great example for this).  Is there any innate significance to age?  Anything that we can feel on a different level than on the level of society?   If anybody has the answer, tell me because I couldn’t think of anything (or maybe I fell asleep before I could figure out the answer).