Can someone explain to me the ending of the Fifteen music video??

The girl at the end is just a normal teenage girl. Kinda like us? Taylor looked at that girl like she knew exactly what she was going to go through. So Taylor is singing her song as sort of warning to teenage girls going through highschool. Warning them/us that it is gonna be tough, but even if it is, it's not the end of the world, it's just highschool, one chapter of your life.

As for the coat and the umbrella, I think they're showing a older more experienced side of Taylor. To show that she went through it and she came out stronger.