This is really interesting thread that's gone far beyond the original idea.
As an adult who lives with a parent and who knows several others who do the same I can offer broad perspective on this. I would say that, yes, living with your parents can be like a second childhood, but that statement needs to be qualified since that, even in this situation, there are various degrees of 'independence'.
Some people in this category are actually vary mature in every sense of the word and are either living at home to take care of their family or due to the ♥♥♥♥♥♥ economy that has been foisted upon them.
Then on the other extreme you have guys (yes, they are always male in my experience) who may or may not have a job who simply play video games all day and hang out with their male friends (if they had it together enough to get girlfriends they might be able to afford an apartment of their own). I think that Steve was probably referring to these types when he made that statement.
Between these two extremes, however, there are actually a lot of degrees of independence.
I have lived with my mother most of my life but it's not the same as it was when I was a child. I prepare my own meals, do my own laundry, and pay for my own things. In exchange for living in her house my mom expects me to refrain from doing certain activities in it that she would not feel comfortable with (such as having sex or hosting loud parties -- though this is no issue for me since I have no interest in either). So while I certainly don't have the same freedom that an adult who is living alone would enjoy, neither do I feel like a child. It's like living an ANY group situation - you compromise with your living companions. In fact, if I did have the money to afford my own place I think I would probably choose to buy a home and invite my mother to live with me.
Some people might see this situation as unhealthy or 'arrested'. But while I am willing to admit that it is far from ideal, I don't believe it's the same as childhood. I once had the idea that it would be really 'manly' of me to move out and live on the streets until I found a job and apartment. After 3 weeks of being homeless in San Francisco I discovered that voluntarily living on the streets when I could have been living with my mom had not made more independent, more motivated, and it certainly had not made me more employable. Having realized this I promptly came home and actually got a job a short time later.
If someone has no other alternative than homelessness then there really is no shame in living with parents and no need for regret either since the only alternative would be worse. It may not be conductive to total happiness but that doesn't make those who are living in this situation big babies; it just makes them adults who must compromise some of their desires to suit their parent's wishes. Does it cut in on your self-expression and lifestyle? Yes, but so does ANY type of co-habitation. Roommates also must compromise with one another. So must spouses. But just because it requires some loss of independence doesn't make it unhealthy or regressive.