can anyone tell what age you should have the first puppy hair cut

Well it depends on the breed you have, is it a breed that needs a trimming? or is it one that just needs a daily brush ?

If you have a puppy that needs regular trimming, then make an appointment with your recommended groomer as soon as the pup is protected against the usual diseases, so the the groomer can train it to be used to the noises and handling that will need to become part of its regular life.
If it's not a breed that needs regular trimming, then if you make an appointment with your recommended groomer, they will show you the correct way to handle your dog's coat.

it is a yorkie

Then it doesn't need clipping.  Just teach him to stand still while you use a slicker brush (not one that the pet shop sells with bristles on one side and knob pins on the other!) and then a medium metal comb to check that you haven't missed any knots.  It'll take you 2 minutes each day.
The only trimming a Yorkie needs is under the tail for hygiene as it gets older, it's feet and maybe 1" taken off the length as it grows.  By the time he needs that, you will have him standing still so that you can just snip it yourself! :)