I love everything about this image. Your style is consistent all the way through the image and the anatomy and aging of the two of them is accurate and realistic even with an unrealistic style.
The use of the colours and the floor as the stages of life change from youth to spring to autumn then back to the winter is a very nice touch, seeing that despite all their travels they end up back home together in the end.
The only thing I can really offer critique as such on is that the colours all gradient except for in the last two stages, Elderly stage look a little too 'bright' as the colours in the rest all gradient along and then it jumps back to stark blue and stands out as though it is unfinished. This I think would be fine if you did not have the portrait on the end but it still works wonderfully.
Though that last bit with Katara alone... man, I still have tears on my cheeks. It's a very nostalgic and happy kind of sadness and I feel that she is sat remembering her brother and their times together.
I really do love that even if you did not understand who the characters were there is still a beautiful story of life and friendship here that is translated beyond words and will touch everyone.
All together a beautiful picture that really does get you