My oldest just went off to college. The parents are 40 and 41. When will I be able to start the new generation? How much longer do I have to wait (aproxx. time)? And now do I start the new generation?

You will get the chance to start the next generation after one of the parent's dies. At that time, you will be able to choose to continue playing the living parent alone in the house or transition to the next generation. When that time comes, just make sure you read the screen that pops up and you'll know what you need to do to move on to the next generation.

As far as how much longer, I really couldn't say as they die at various ages, but most of them seem to be in their 60's at least. It depends on a variety of factors though, such as whether you bought them regular food or organic food, do they exercise often, do they have a well-balanced life between working and playing, etc. Also, if they are sick I think that may cause them to die sooner too, but I don't know as all of mine have died from old age instead of sickness.

I had a couple who only had 2 children of their own. They were in their late 40s when the last went to college. I just kept them well fed and exercised. Then a little girl showed up to be adopted.