I'm not sure if this situation applies, but my son just left for a 2-yr church mission where he will be on a bike the whole time. I called USAA twice and asked if I can drop him off my policy. They told me no because:
1) It would make it really hard for him to get insurance when he comes back (lapse of coverage)
2) As long as we are responsible for him I had to keep him insured
3) That if he was in an accident (on his bike), they would still cover him.
4) That if he cause an accident (on his bike) it would protect our assets.
5) Etc., etc., etc.
The second guy sold me a non-owners policy that would save me a bit off having him fully covered, but I was still upset that they wouldn't let me drop him and told them I would be looking into this with other companies. After talking to other missionary parents, I found that they all suggested that I drop him off my policy. I called back and got a supervisor. She told me that I had been misinformed and that she would drop him off my insurance, cancel the non-owners policy and get me a refund. She spent about 45 minutes trying to undo what her coworkers had done.
Anyway, yes, it is possible to drop your adult kids off your policy and get them on later.