Hamster acting strange

Question
I have a fancy black bear hamster. I've had him for about 3 weeks now. When I first got him I thought he was a girl, but I recently found out otherwise when I saw his testicles. I have a few questions now:
1. How old are boy hamsters when their testicles start to show? The PETCO workers said that they didn't know how old it was, but it was at least old enough to leave its mom.....they didn't know it's gender either :(

2. Ever since he started showing he has been acting strange. He is awake a lot in the day time walking around and eating. He has also been going to the bath room a lot. Today, he spent a lot of time just sitting up on its butt. Is this normal or is he sick?

3. I read on a petco sheet that my hamster will grow to be 6 inches long. How long does it take for a hamster to be full grown or an adult?

Thank you.....sorry if some of my questions are a bit out of your specified area of expertise.

Answer
Hello Mary.  In my experience I do not trust the Petco employees too much unless they themselves have had hamsters for pets before, many times they haven't.  I've been told several times by Petco employees that a hamster is a female, when clearly it was a male or vice versa, the employees rely heavily on what the sign says on their display.

Baby hamster boys only need to be 3-4 weeks old before their testicles start to show. So you bought him when he was just barely showing.  They are old enough to leave their mothers at 3 weeks but I usually recommend keeping them with their mom until they are 5 weeks, just so they learn where to get food and water.

The baby is probably just confused so he is exploring his new found territory.  Hamsters will stand up for a few seconds to a minute trying to figure out what certain sounds or smells are, he is just acting normal.

The Black hamsters tend to grow up to six inches (which is pretty big for hamsters) and you can expect him to reach this length by no sooner than 6-7 months.  This is if he is a fast growing boy, it usually varies kind of like it varies for how fast a human will grow.

I hope this helps answer your questions,  Take care Mary!