There are 15 year old grandmasters. They have good ratings. Â Nearly everyone looks bad in comparison to those phenoms.
On the other hand, there is your rating. Is it up over the last year? Â Yes it is. Your rating is 1400 today versus 1257 a year ago.
Are you better than the average player here?  Look at this graph. http://www.chess.com/echess/players  The average online rating is 1343. You are better than the average.
Then it is a good rating.
What would you say is a good rating for a 15 year old?