Even though there may be "emotionally mature" 20 year olds out there (which sure there are,) that doesn't mean that a 20 year age gap works. Because it's more than just being "emotionally mature" in a relationship. The bigger the gab, the harder it is to keep a relationship going just due to pure experience in life. Plus, at 40 the physical issues begin, even people who are really healthy your health is much more likely to suddenly turn. I have a 50 year old boss who acts like she's in her 30s running around, but she's discovering that although she's been very healthy for her whole life her body is starting to turn on her.

Lastly, I agree with the first post. In most cases it has nothing to do with emotional maturity and everything to do with emotional immaturity and wanting to "go back in time." And seeing women only as objects, not people.