Trane Useful Life

Hope I'm posting this in the correct forum.

I do commercial property inspections, and as part of my job I need to estimate a useful life for HVAC systems. I recently came across several Trane SCWD units, and havent been able to find much data on their useful lives. My Trane contact is out on vacation, so I hoped someone on here could help me.

The units are 80 tons manufactured in 1996. Cooling tower on the roof. Property maintenance tells me they receive alot of attention. Any ideas?
Thanks,
Jay

Maintenance contracts- We have several buildings with these units and they still run like a top, if properly taken care of you should have to change the cooling tower out before the units.