I feel compelled to point out that almost all of those numbers are completely incorrect overestimates.
There’s 2.3 billion email users worldwide, and the average mailbox stores 8,024 messages. versus 150 billion emails each day These numbers do indeed fail the sniff test. This would mean that the average age of a mailbox would only be ~120 days (150B/2.3B daily mails/user = ~65, 8000/65 user account size/dail user mails= ~120 days). That's a mere four months. Of course, this is only if people aren't regularly deleting…
If you look at the growth in email users over time (~6%/year), most accounts are probably > 1 year old. I'd assume the disparity comes from people deleting messages.