Rick.
I'm not a parent, (thank God!) but I am a 23 yr. old and I do still live with my parents, so maybe I can give you some insight from the "child's" point of view.
I lived with my parents all throughout highschool... after graduation I decided that I wanted to attend a University either in Birmingham, AL or Lakeland, FL... as I was making this decision my parents actually approached me with the idea of living at home for two more years and attending a junior college here in town in order to save money (tuition, living expenses, etc.) and also to get "grounded" before I moved off "on my own"...
After I graduated with my associates degree,I was 19 at the time, I still felt confident that I wanted to attend a University away from home... I spent the next two years 500 miles away from home, my parents, and my friends... but I was ready. I watched as a good majority of the new students who were "on their own" directly out of high school could not handle their new found freedom and soon dropped out or flunked out...
I was thankful that my parents took the time, even after I graduated high school, to model the lifestyle of a responsible adult.
After I graduated with my four year, I moved back closer to home... about an hour and a half away, and begun to work as a youth minister in a small school district. This is where I see God's sense of humor in His sense of timing... because when I became the most involved in the church I had ever been this is also right around the time I began to question the motives and teachings of the church.
I was there for about 6 months before I resigned. I just couldn't teach/model something that I didn't believe to be total truth. About 5 months after my 22nd birthday I found myself back at mom and dad's place... living in my old room, trying to figure out what in the world I was suppose to do now?
From the parent's standpoint, I imagine it is difficult to determine how long a child should stay living at home, but let me assure you... I, as the child, was more then a little embarrassed that I had ended up back at home. I felt like a 22 yr. old living at home after receiving a 4yr. degree was a sign of failure or lack of initiative...
I took the next six months to work and save. I enrolled in a Master's program at a local University and began to search for and apply to other Universities that I was interested in doing master's work at... shortly after I found an awesome job that I was totally unqualified for, yet for some reason God saw fit to place me in...
Last week, I just received my acceptance letter to a great school in TN and am now, even this morning makign arrangement to move "out on my own" again.
I say all this not to simply bore you with my personal story, but to show you that their are a mutiplicity of reasons that a child may still live at home that are all determined by thousands, if not millions, of different factors. Some are good, some are reasonable, some are bad... my advice to you is to sit down with your child (whether step or not) and talk with them about their goals for the next 3-5 years. Then talk with them (even if you don't know alot) about how they can reach those goals...
The situation that children are in now is crazy! Children are reaching sexual maturity at an extremely rapid rate and at the same time, adolescence is being stretched out because of the cost of living is far above what most 20 yr. old's can dream to make.
Within your realm of capabilities offer to help your children set and meet their goals... that may mean them living at your place for another few years... but as long as they are willing to abide by your rules and pull their weight (maybe even rent?) is that a problem?
Just a couple of thoughts, but what do I know... I'm just a 23 yr. old kid, right? =)
Hope this helps.
Grace and Peace.
Josh