It is primarily heat that kills transformers, not age in years.
Provided that the transformer is in a well ventilated area, not overloaded, or otherwise abused, then I would expect a life of AT LEAST 30 years.
The load on POCO transformers is very variable, and they often survive longer in cold climates.
In a cold climate, the peak oload is likely to be on the coldest day of the year due to electric heating, the cold ambient partly compensates for the high loading.
In a hot climate, the maximum load is likely to be on the hottest day of the year due to widespread use of air conditioning. Unless the transformer is generously sized, this can contribute to a shorter life.
As the years go by, there is a slightly increased risk of failure.
I would not expect the efficiency to decline with age.