I have a mountain bike with semi-slicks. I've been doing all city street riding....nice smooth surfaces. I've been measured for my bike. I have proper leg extension, comfortable back position, and can probably ride an hour without any problems.

However, once I stop riding, I get pain in my knee joints. I only started riding about a week ago. Prior to that it was about 25 years since I last rode.

Any suggestions as to why I'm getting the knee pain?

it is not uncommon to have inflammation in your knees after any kind of workout (cycling, running, hiking, etc). is the pain just a dull ache or is it really UNBEARABLE? there could be many things going on...but i'm guessing that since you just started riding your body isn;t accustomed to the stress yet. do you stretch? how hard are you riding? are you spinning or mashing? are you using spd (clipless) or platforms?

The pain is just a dull ache and goes away after about an hour. I don't ride hard and I don't stretch before or after I ride. I'm not using clips. I think maybe my body is still trying to get used to riding. How long will it take?

Ice and Advil...

"I don't ride hard and I don't stretch before or after I ride. "

While your muscles and lungs may not think they're pushing it, your knees think otherwise. Might be a good idea to do some stretching before and after. Make sure your heels can't touch the pedals when they're at the bottom and push on the pedals with your toes, not the ball of your foot or your heels.

well i'm no doctor, and as far as knowing how long it will take for your body to get used to the stresses of riding -- that's a difficult question. it could get better soon, but it might also get worse! knee problems of the overuse variety are quite common in cyclists. just be careful not to overdo it it in the beginning, and as someone else mentioned, ice and ibuprofien are good remedies. ice them immediately after you get back from your ride.

Mashing the larger gears instead of spinning the smaller gears? Using too large of a gear at a low cadence can cause knee pain.

check this out:

https://www.cptips.com/knee.htm

If you are riding for an hour and not doing any warm up or easy riding to get going with it, try the old timers trick to bring some blood into the knees.

Before you head out for your ride, massage some Tiger BAlm into the knees, all around, sides and back too, not just the front.

It does a wonderful job of "loosening" up the knee.