Plenty of aircraft have served beyond 30 years. It's not unusual. Age of a design alone is certainly not an indicator of usefulness: just look at the Dakotas that still earn money in various parts of the world. The Hunter was in squadron service for over 40 years and the Canberra, I believe, 50. Properly supported, funded and maintained, any aircraft can last practically forever. If it's a good design and does it's job well, that's no bad thing.
I heard somewhere that when they reach their forecast out of service dates, some lumps of B-52 and Nimrod, will have been serving for the best part of 100 years (although perhaps in the same manner as my grandfather's broom).
Incidentally, didn't the RAF recently have to lease back the Canberras that they had just auctioned off, because nothing else available in the inventory could do the job of the Canberra - a late 1940s design.
