One does not necessarily have to be 18. Age is nothing but a number. Once someone reaches the maturity level needed to live alone, its in their right to do so. People need to start being independent because once their parents aren't on earth, they'll need to be self sufficient.
I left home at 17. I struggled a bit at first, but soon learnt to take care of myself. However, I had no kids, was in college and had my now husband for support,
I think for the purposes of education it is fine to stay at home and focus on your assignments etc, but young couples should not live together in their parents home simply because it isn't fair on other family members to have to tolerate a stranger and put up with privacy issues.
If they are mature enough to hold down a relationship and want to sleep with their partner each night then they should find their own place,
Forcing kids to leave right at 18 would be problematic because most people turn 18 during their final year of high school. But once a child graduates, I think they should be out of the house within a year. Families will always love and support each other, but at some point the child must become an adult and lead their own life.
I think children should have to stay with their parents because ,parents needs support of their children too when they grow..They nourished us from our childhood ,they help us when we are in need ..And no body care and love us more then parents..So why we leave them alone when they need our us most?..They don't need more then time from us..All their need from us is time and respect..So give them most of it..