My ds isn't 18 yet -- got 1 1/2 yrs yet -- but our lives have been gradually changing anyway. He will be attending f/t college in the fall (less than a month! ack!) and that is a HUGE change for this formerly homeschooling mom. It was a transition, though -- he went pt to a charter the last two years, and spring semester pt to college, with very little homeschooling added on. But I am now going to be mostly out of the loop (he will be living at home, though) -- and I am at a weird place in my life where I am trying to figure out what to do with my life again. I don't want to go back to what I did years ago (programming -- would have to learn too much over, and really it was never a passion, just a job), but not sure what direction to move.
To add injury to insult, I hurt myself while taking him to a 3 week summer program, and am now in physical therapy. So all the things I had planned to do to get myself organized is delayed. (I was totally unable to do things -- even tie my shoes -- for a bit.)
I think some of the milestones that will make additional impacts are if/when he starts driving (he is not interested at the moment, despite being old enough), him graduating/getting a job, and moving out.
But even without those, I have felt a major shift in our lives already.