Agree with Ian, It depends on the aviation ploicy of the company wanting the aircraft on contract, they will usually be advised (in the case of oil and gas) by their Aviation Rep as to what the criteria is. If following OGP, they will want as a minimum, the requirements as stated in the OGP 390 AMG, OGP does not state age limitations. I have worked extensivley in Asia where the advise to the client was nothing over 10 years old (based on airframe availability in the region), that said we did have cases whereby the airframes were older, however we also ensured that the correct maintenance regimes were in place (including spares) to support the operation, and it was usually monitored via an onsite rep. Hope this helps.