If you need to find out when a specific user mailbox was created In Exchange 2016,2013, 2010, 2007 you can use the PowerShell cmdlet below:
Type the cmdlet below:
Get-Mailbox administrator | Select-Object whencreatedUTC
To find User creation date In active Directory visit the article Find User Creation Date In Active Directory.
Proper query is:
Get-Mailbox userid | Select-Object whenmailboxcreat*
This will give only the AD account creation. To check the MBX date need to run Get-Mailbox | ft Name,WhenMailboxCreated
You cna also put an identity of looking for a specific mbx