I guess the best question for this question is whether a person is able to write instead of age. It just takes a few seconds to set up an account to write a blog.
If a person is able to write a lot then it is really good to start young. This will develop the necessary skills for a person to advance in his writing skill. Blog writing is a skill that a person needs to practice and improve. It also depends how much expertise one has towards writing a certain topic.
Choose one that is really fun to write about. It will be fun.