Can anyone give me an idea of the age that a puppy's testicles come down, is it different for different breeds?

They should be noticeable (not fully descended, naturally) by 8 weeks.

I have a 12 week old puppy, one is down and very visable the other the vet said he can feel, so is it likely to come down?  The vet thought not but a friends dog's didn't come down until 17 weeks.

Try not to worry, it can appear at a later date.  My friends dog had his 2nd one come down at 12 months.

Was a bit worried about my two boys from my last litter as they hadn't dropped at 8 weeks, one of the dogs kept on popping up and down and the other ones I could definitely feel one but the other wasn't to be found.  By 12 weeks they both had them down and they are definitely there now LOL! 

Anton when he was younger they were really tiny, they were there but you could hardly feel them, can't say that now though!