I'd say 24-25 is a good "absolute cutoff". I mean, honestly, get on with your life. To spooge's point - sure, it makes more financial sense to live at home if you can until you're ready to buy a house. Why not stay a little longer to save up a bigger down payment and same some interest $$$.
However, you learn to fly by jumping out of the nest. There's a lot more to living on your own (in terms of growing up) then just paying rent, and I think it's valuable to give it a shot as soon as you're ready. Having parents to fall back on is a bonus for when you screw up.
Of course, this is coming from my point of view - I was on my own at 16.