i did the chipping training course last year and since then i've chipped all my puppies at 7 weeks. so they are done and dusted for their new owners to collect at 8 weeks. i've also chipped a few adults since then and i must say that the younger the dog the easier it is for the dog. all my pups just sit there and a few have been asleep during the jab. the adults i've done often have an idea what's going on and react to you more than the jab.
all of my dogs and cats are chipped as are all of my freinds and families dogs. for peace of mind i would recommend everyone has their pets chipped.