How old should you be to use blog?

Each blogging platform has it's own legal definitions. The common age is 13.

Blogger.com: age 13

Wordpress.com: age 13
some website let tweens make blogs..... (tweens are 7 to 15 yr olds)
To own a blog you have to be 13 years or older.
Any age really it depends what blog site you create your blog on