When to give children the boot ?

I think in a healthy family, it's reasonable to permit adult children to continue living in the home until they are adequately prepared to leave. 

There really isn't a one size fits all. As long as they're pulling their weight and contributing in an age appropriate way, there's no reason to boot them out. Some people live with their parents for decades and it's okay. However, they should be helping out with cooking/cleaning/laundry and be able to drive themselves to work, etc. Don't pay their cell phone bill or car insurance. The goal is to get them to be self sufficient. 

When I was in my early 20s, my mom had high expectations of me. She always put a lot of pressure on me to go to college/get into a high paying profession -OR- find a husband young because that's the path she took. Needless to say her getting into my personal life caused a toxic relationship. Don't micromanage your kids and let them express their individuality. That's what gives them the energy to do something productive. They have to live their own lives. 

21+, unless the son or daughter is disabled, in college, or paying rent.

When they have graduated college and have a job

When they turn 18, ours had 3 choices, join the military , go to college or get a job and move out.    They have to go in order to grow is our stand on it.   So far it has worked well for us as both of our kids have good careers and making their own way.

When theyre able to support themselves. As long as theyre trying to support themselves, theyre your children, and you shouldnt kick them to the street to be homeless. 

This is a cultural thing. In the American culture, it is very common to "kick" out teens who have graduated high school and are adults. In other countries, it is more common for people to leave their parent's home once they find a spouse. It depends on what you want for your child. My suggestion would be to help them as much as possible without holding their hand. 
Young adults need support because they are not ready for life yet. Let them have a job and pay a small rent for staying at your home if need be, and let them pay bills and other expenses without them having to worry about a mortgage or a huge rent. The purpose of raising a child is to get them ready for the world. Just because someone turns a certain age doesn't mean they are ready for the world. They are ready once they can sustain themselves and that is different for everyone. 

You should never give your children "the boot". They should only move out when they are ready, and that will vary greatly for different people.
Obviously if your child is disabled, the answer to that is never.
If you aren't prepared to still be changing your child's diapers when they're 50 years old, you have no business being a parent.

I was 18.  A couple months after i turned 18...i had to finish the school year and move all my stuff...took a while.

Why do you need a particular age standard? You do what you do and I'll do what I do. It's all good.