Empty nest syndrome.
That is why I planned to have so many kids. It will be a while before all my kids are adults and by the time the youngest (who is 7 now) is an adult, I will have a flurry of grandchildren. There will always be kids in the house. (I am just having a glad break from the diaper stage for the time being
) My adult children are either working with us or they are away at college/university. Sometimes they are travelling abroad.
Yes, I miss the my adult kids when they are away, but we keep in touch regularly. That is the beauty of social networks and skype. Was just chatting with my daughter who is at University in Australia. My eldest son (the second eldest) is 'home' from College. He was away for 2years. His little brother is sticking to him like glue and will not let him out of his sight, bless him.
I agree with Jillio, it is a good time to start doing something you didn't have the chance to when your kids were at home.