Surely it must be possible to chip at eight weeks as I know of several people who chip their kittens before sale & when looking into other animals that are chipped when a rescue parrot we had in for rehoming came with his chip details I found that rats are chipped.

Can only be a good thing and another step in the right direction lets hope that decent breeds add themselves to the chips along with new owners when registering details, something many decent breeders already do.

Jet and Copper were both chipped by the breeder before I got them at 8 weeks.

A lot of the comments we see on here are the same as those made when the DVLA announced a new database. In the early days, it may have appeared that those denouncing it were being proved right, but nowadays it's a brave driver who risks driving around without licence, tax or insurance.

The microchipping database will take time to get right. Once it's sorted out I hope they will start enforcing with a vengeance. I'd love, in time, a lot more information than current owner's name and address to be added/held on that database. So very, very useful if they ever do get to that point

Tiny things like captive-bred released re introductory harvest mice are micro chipped for wildlife conservation studies. They must be very much smaller than even Chi pups being only 6 grams when fully grown!

I am not aware of a minimum age for chipping cats or dogs?

I have heard of vets advising people to wait until the neutering GA to have their animals chipped, as they won't feel it! That seems to rather defeat the purpose! What if the pet gets lost or stolen before that, or they don't intend to neuter?

IME microchipping seems to affect animals to the same degree as a vaccine, despite being a wider needle. Some clearly feel it, some doesn't, as with vaccine injections.

Or one that's driving someone else's car or has lifted an appropriate numberplate.

Yep, and we still have murderers and burglars and goodness knows what else despite the police and DNA databases. Think we'd have a lot more of 'em though if we didn't have Laws to protect us

Well people who can microchip cats/dogs, can't microchip mice..so thats not even relevant!

There is no minimum age, but I remember being at the training and being told about 8 weeks is ok, but people like to wait a little longer for smaller breeds/kittens, as there is more body bulk!
But I guess it's a personal thing, to the person who is doing the microchip, and seeing how small my boys were at even 14 weeks, I decided not to do it for another month.

Can't in what sense? Or don't? Which is different.

The relevance is/was that the conversation appeared to be concerned with size/weight considerations for chipping?

You are of course entitled to decide what age/size you feel happy to have your dog(s) done.

CAN'T.
We are trained and allowed to do Cats, Dogs, Rabbits and ferrets.
Anything else would come under different training/qualification I guess.