Usually i would say pup, but i managed to get a 23 month old beddy x whippet and he is 100% on the button. Apart from being hard mouthed (which i was told when i went to see him run) he is all there. So you can if your lucky, and under the right circumstances find a decent older dog. Sometimes there are circumstances when someone needs to part with their dog, maybe cash, or personal reasons etc... which is a sad part of life. But i can also see that there are other reasons not to buy an older dog such as getting bad habits etc..
In my opinion, if you are offered a dog, go and see it work,