Holden Astra 2000: Should the timing belt be replaced?

The Holden Astra City I bought new in 2000 has done 55,000 km and I've only just learnt about the timing belt issue. Should I consider the 11-year age of the car rather than the kilometres when deciding whether to replace the timing belt? It also has some strange variation in the revs, when started it revs up to 1900 before dropping back to 1000 and then settling at 900 revs, all without my foot being on the accelerator. Then, once it settles the red ?oil? lamp light flickers on until the revs rise above 1000 rpm again. What is wrong?