Gmail (Google email) is disabled on student Google Apps for Education accounts for grades K - 6.
Gmail is enabled for 7th - 12th graders.
Personal Google accounts have age restrictions of 13 and older only.
Google accounts are restricted to anyone under the age of 13 (website) even with parent permission. Though, nothing prevents them from making one and lying about their birthday.
Many parents do allow their children to use email or other messaging services.
While this is against company policies, many consider the age of 13 as very late to use email and messaging.