We continually buy new server stock, so it is likely that the oldest a server will be is a few months. All hardware is burned in when we get it though, as even new hardware can have faults so to help ensure we deploy stable machines, we burn the hardware in for 24 hours and then re-stock it in a different set of shelves.
We also stock full replacement parts and have a 2 Hour Hardware Replacement SLA! This is one of the strongest in the industry.
www.pacificrack.com/agreement.html