How to know When I Created My Google Account ?

Here's a simple way to find when you've created your Gmail account. You can always find the oldest message from your account by visiting this page https://mail.google.com/mail/#all/p1000000: you'll get an error message and Gmail will redirect to a page that shows the oldest messages from your account. The problem is that this is not a reliable way to determine when the account was created: maybe you've deleted some messages, maybe you've imported messages from other accounts.

If you've joined Google+, you can quickly find the exact date. Go to Google Takeout, click "Transfer your Google+ connections to another account" (don't worry, you don't have to do that), enter your password and you can see the date when you've created your Gmail account in the blue box at the bottom of the page.

Why would you need this information? For example, Google's account recovery form includes a question about this. "If you find yourself locked out of your account and none of your other recovery options work, your last option is to visit our password-assistance page and fill out our Account Recovery form. (...) Since Google doesn't collect a lot of information about you when you sign up for an account, we'll ask you questions like when you created your account, what Google services you use, and who you email frequently (if you use Gmail)."

I haven't tested this, but I assume this trick also works for non-Gmail Google accounts.