Overview older than 30 Days:
Get-MobileDevice | Get-MobileDeviceStatistics | where {$_.LastSyncAttemptTime -lt (get-date).adddays(-30)} | out-gridview
Delete active sync devices older then 30 days
Get-MobileDevice | Get-MobileDeviceStatistics | where {$_.LastSyncAttemptTime -lt (get-date).adddays(-30)} | Remove-MobileDevice -Confirm:$false