I went to the pet store a few days before I got my first rat, and the lady told me that the one I held was about 6 weeks old. I went back a few days later, and that litter was gone, but they had new ones. They looked about the same size, so I assumed he was about 6 weeks old. He didn't have any grown testicles, just little bumps. They started growing in around a week or so later. I was just looking at a "goolies" post and someone had a pic of a 4 week old rat's goolies. They were much bigger than Templeton's goolies when I got him at supposedly 6 weeks. Now I'm confused about how old Templeton really was. When do testicles start growing in? If they start growing at before 4 weeks, then Templeton was several weeks younger than I thought he was, and entirely too young to be away from his mom.  Stupid pet store people.

Rats usually drop their balls at about four weeks.  To be honest, you can't trust pet stores.  I knew nothing about what rats looked like as they grow, so I believed the pet store when they told me he was weaned and ready to go.  I later found out that one of my first boys was only two weeks old when I brought him home.  His eyes had just opened and they saw him nibbling out of the food dish.  They just assumed that meant he was on solid foods and therefore weaned.  A couple years later, I asked how old a couple boys were, at that shop.  They said they were six weeks.  Now that one of my girls had an accidental litter and I've seen how they grow, those "six week olds," were the size of mine at about four weeks.

Or he might have been terrified and retracted his goolies. How long does it take for them to come back out when they do that?

3.5 weeks is when I normally start seeing them.  By 4 weeks-ish they are definitely testicles.

Yeah sounds like your little boy was weaned at 2.5-3 weeks old when you got him....Grrrr.