L.B.
Lying should not be an acceptable behaviour at any age. All kids try to lie, at least once, but if it's caught and disciplined properly it shouldn't happen continually.
Kids typically lie if they don't want you to know something or are trying to hide something. If they are lying for things they wouldn't get into trouble for in the first place, my first thought would be that the child has developed a bad habit. It's a habit that needs to be broken. Perpetual lying is not a good thing and the child needs to understand that.
Hope that helps.