"What is the normal age for the kids to get their own email account?"
If only emailing with grandparents or family members, there are students as young as 1st grade who I know of having email addresses.
If opening it up to friends, I would advise waiting until 5th grade. Children at that age seem to grasp what is “appropriate” and what isn’t....not to say they always DO what is appropriate...but at least they can identify the category! ;-)
Whenever you decide it is "the right time," set expectations, have conversations, and practice, practice, practice. Since the motivation behind sending an email is communication, children should be taught proper communication skills. Emails should have a salutation and closing and always include their best age-appropriate spelling. Children should get in the habit of re-reading what they wrote and signing their name before pressing send. I guarantee, the novelty of sending emails quickly fades when all of these elements need to be included!
Another good rule is, "All media that leaves our house must be approved by a parent." Children often want to take and send photos or videos to friends and family. A parent should be asked each time any media is going to be shared. Sometimes the photo is harmless (the family pet) but the background might be something you don't necessarily want shared (mom in her pajamas on Saturday morning). Sometimes there is no real purpose for the media being sent - "Tell me does your friend want to see a recording of Taking Tom singing a song?" Sometimes silly is ok, other times it is not. These are great opportunities for conversations about purpose and audience.
In my family, both of my children (ages 7 and 10) have email addresses set up that we use for signing up for online accounts, apple IDs for their ipod, etc. They do not know their passwords, though. Those I am keeping until I allow them to login and use their email accounts themselves.
A few months ago, my oldest (5th grade/age 10) was allowed to begin using email. The account is set up so that all emails are cc'd into my email account - we see every email that comes in. We also have frequent conversations about the content and quality of emails sent and received.
That said, before giving a child an email address I strongly recommend having a conversation about your family expectations with regard to technology. I would also encourage you to write up and post in a prominent location, a list of rules/guidelines/agreements for your family.
Visit theonlinemom.com or commonsensemedia.org and look through their articles for more information about children and email accounts.