We live in a six-unit apartment building, and all units are served by a
single water heater.

Last month we had a sudden increase -- about 25 percent -- in gas usage.
Emphasize, USAGE, not price -- which also increased.

The water heater is around 10 years old. Do such heaters lose efficiency
with age?

There is no other gas appliance on this meter.

Any guidance or suggestions welcome.