What is the normal or average life expectancy of a Transformer ?

When a Transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal life
expectancy is about 20 years. The ANSI / IEEE basic loading conditions for Transformer are:
 
i. The Transformer is continuously loaded at rated kVA (kilo Volt Ampere) and rated Voltages (Transformer must be operated at the rated Voltage and kVA)
 
ii. The average temperature of the ambient air during any 24-hour period is equal to 30°C (86 °F) and at no time exceeds 40°C (104 °F).
 
iii. The height where the transformer is installed,
does not above 3300 feet  or 1000 meters.