There is no set age as far as I know. In South Africa our registry has announced that any puppies registered after September 2010 must be microchipped so that means theay are being chipped at a young age
my vet told me to wait becasue he said the needle is big so why worry about it when she is so young. I had it done last weekend when she was 7 months old and they predicted she would cry but she never even noticed the needle! Mine tends to run off at any chance she gets so i have chipped her now at this lively age
hope that helps!