I've been pondering about this for some time now. I've also searched the internet about it but all the help that I can get is all but outdated.
Actually, what we are looking for is always there and is barely unnoticeable unless you click it.
Today, I finally found where you can see the exact date when your Yahoo email was created and it's inside "Privacy Dashboard".
Yes, inside the "Privacy Dashboard" of your Yahoo mail account.
So, how do you go to "Privacy Dashboard"? It's a bit of simple actually. Just follow the following instructions (step-by-step tutorial and video tutorial).
STEP 1. Click your account name then click "Account info". IMPORTANT: If you are asked to click "Next" or enter your password just do so.
STEP 2. Click "Preferences". IMPORTANT: If you are asked to click "Next" or enter your password just do so.
STEP 3. Click "Privacy Dashboard". Wait for it to load and you should see an entry like the screenshot below.
STEP 4. Wait for it to load and you should see an entry "Member since" like the screenshot below.