At what age should the testicles be there when palpated? My vet only felt one at his eight week checkup.

Immediately after you place them in a pet home.

Seriously, I had one that didn't have two at 8 weeks, pick puppy in the litter. Sold him to a pet home and Voila! they were down by the time they went to their vet within three days of the sale. He never had two since I started checking for them at a couple weeks.

I've also had a male I sold drop the other one at 6 months or so.

I my program we have many that do not have 2 dropped at 6 weeks yet by the time they reach 14 weeks in my home or a Pet Home they are fine. If he is one you are keeping I would hold out a bit.

That is so hard to say. You can feel two at 5 weeks. Down some by 8 for sure, but have had other tiny for a long time. But both there. All of mine were both down at same time. Not much help.

When I get a puppy that does not have both testicles down by 8 weeks, I place him as a pet. He will transmit that delayed drop to his kids. It is a hazard to have a drop dead gorgeous puppy missing a testicle, and keep on waiting and waiting. A lot of times they don't drop. JMO

My only experience with a late testicle was a boy whose 2nd one did not appear until he was 8 1/2 months old. Testicle grew to completely normal size & I never had any delays with testes on any of his puppies