Powershell - When was a mailbox created.

Super easy...

Get-Mailbox UserName |Select WhenMailBoxCreated

Note:
WhenMailboxCreated       refers to the mailbox
WhenCreated          refers to the AD Account.

Caveat
Prior to Exchange 2007 this variable did not exist, thus after an upgrade from prior versions it will hold the date of the migration.

Cheers!