We have a house built approx 1999 with wired interlinked smoke detectors. These have 9v batteries which are replaced regularly. I believe that if either alarm senses smoke, both are activated.   Do these alarms have a fixed lifespan or are they still ok if they seem to test ok ?

All smoke alarms should have an expiry date printed on them. The average life span for a smoke alarm is 10 years.

Personally, I would not be worried if I changed the smoke alarm by year 11, but I would say 25 years is too long.