I agree about the long term health effects of Mars gravity. I think with adults you'll probably see some bone density loss, similar to what happens to astronaut

I agree about the long term health effects of Mars gravity. I think with adults you'll probably see some bone density loss, similar to what happens to astronauts on long space missions. But arguably that will be offset by gravity at 39% of Earth's. The bigger concern I have is for children that are born on Mars -- if you want to have a self-sustaining population there, you're going to be raising children there, and physical growth in a low-g environment isn't something we can test in the timeframe Musk has in mind.