Is it normal to feel sadness and grief over your children growing up, knowing they are going to be gone in a few years?

I had my four children by the time I was 24. My oldest is a senior, while my others are a sophomore, eighth grader and a seventh grader. I feel, at times, that I go through a grieving process more now than ever as they are getting older and time is racing by so incredibly fast.

Is it normal to feel sadness and grief over your children growing up, knowing they are going to be gone in a few years? My oldest just joined the military and leaves after her final year of high school.