I wasn't really able to just stand toe to toe with one until probably my upper teens in level. And now, at 23, my shield maiden can pretty much sit in it's face, block it's attacks, and bash it's head in. Early on, you need to make use of two importsnt things.
First, cover. Even a single tree can be very useful in saving yo from dragon breath. If you get attacked when not ner cover, have the dragon chase you to where there is some. Dragons also act differently around cover and will be forced into taking pot shots at you rather than trying to chomp your face.
Second, help. It's not always easy to get something else and the dragon fighting eachother, but it can do wonders. If you're in/near a town, have the guards help you out.. and let them take the brunt of teh assault. They're stronger than they look and will just come back if they do die, just make sure you get some damage in of your own so you get the soul when it dies. Help out with archery or long range magic, even if those aren't your main skills, but let them do most of the work.