Exchange 2010 can answer when a user's primary mailbox was created.
[PS] C:\WINDOWS\system32>Get-Mailbox finn2617 |select whencreated
WhenCreated
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05-May-15 10:25:26 AM
My question, is how do we find the same info for an Exchange Personal Archive mailbox? And it would have to be WhenCreated, not when it was moved onto its current database, which is what I seem to be finding when I pull MailboxFolderStatistics, per this link.