If a person stays on the lunar surface for too long, their bodies will eventually adapt to the new gravitational environment. This will probably occur about 35 years after being fully operational, or 5 years after the introduction of the VentureStar II.
One–sixth–gee will become the new one–gee. This means that Earth's gravity will become the equivalent of 6–gees for the person who has adapted. Therefore, once adapted, they can never go back to Earth.
For older people, this new gravitational environment will be an ideal situation. Less gravity means less stress on the body and (theoretically) a longer lifespan.
Additionally, most medical procedures, including surgeries, need even a small amount of gravity to work properly. This is why the microgravity environment aboard the space station is ill–suited for permanent residents.
We cannot, however, allow regular colonists on the Moon because they will be there permanently. Births should also be banned since a human cannot grow properly in a lunar gravity environment.
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