I think you can insist an 18 year old does something if they are living at home and you are supporting them. I know some parents find this difficult, especially if there have been no previous problems.
In my experience, boys will not do anything unless they are forced to. My son in particular who is very nearly 18 needs a rocket up his backside to get him to do anything. He needs to be constantly reminded to do the right thing.
I made him get a part time job by forcing him to leave the house when I did in the mornings (in the snow in February), giving him a list of places to go to apply, and a bus ticket. I had to lock the doors and windows to make sure he didn't get back in as I knew he would give up after half an hour. I am sure the manager who employed him was very impressed that he was out looking for a job at 11 am at half term in the freezing cold!
Most parents would feel cruel going to this extreme but I knew I had to, otherwise I would have had a layabout son, as he had not work ethic at all.
He managed to get a good part time job within 3 days and he has been there 4 months now and it is the best thing that could have happened to him.. I now have to "encourage" (in other words force!) him to apply for courses in September as I don't want him lying in bed most of the day, even though he has the part time job. He did nothing at college over the past 2 years and will get disastrous grades in his A levels. He can't work part time forever and needs to do more to put on his CV.
At the end of the day some parents don't want to rock the boat with their adult children. I work with a woman whose son is 19 and has never even had a part time job - she funds everything for him - he does 16 hours a week at uni. She wants him to get a job but as long as she keeps paying for everything he definitely won't.