Life expectancy indirect-fired water heater?

Specific Inspection Topics Plumbing Inspections
1

Anyone know of this?

2

2-12 years… http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/warranty/550141077_0314%20-%20Water%20Heater%20Res%20Warranty%20(Aqua%20Plus).pdf

3

I have never seen it in writing anywhere. I see them 20+ years often in my area, no signs of leaking or exterior deterioration. Problems I see are usually with the threaded connections leaking from an original poor installation.