Here are 2 ways to figure it out.
Firstly, you can head over to your primary inbox and search for the introductory mail you received from the Gmail team. The date and time stamps on the email will tell the date you've created the account.
Now some of don't keep the introductory mail. As soon as the account is created, we delete the emails as we already have a good idea of how Gmail works and about all the features available.
Once you've logged into your account, head over to the mail settings.
Once there, click on the settings for Forwarding and POP/IMAP.
The POP settings, if not changed, will show you the date the account was created. Knowing when you created the account may not seem so important, but it always helps to remember when your account gets locked.