If the low gravity makes peoples' fitness decay over a decade or two, parents can't raise their kids, and in fact kids can't develop either.
Providing radiation shielding in an economical and psychologically benign way seems much more challenging.
The long-term health effects of low gravity are not a solved issue as far as I know: https://www.scientificamerican.com/article/how-does-spending... https://www.wired.com/2014/02/happens-body-mars/
I asked about gravity and not the (many) other issues because we have at least some hints on how to handle say radiation shielding, however we have no idea how to change gravity if it does indeed prove to be a blocking point for permanent settlements.
By all means, file this under "possible, as-yet unquantified risk". It's certainly conceivable that there is some killer disease the crops up in 1/3 gravity if you don't exercise daily. But right now I don't think we have any data that's inconsistent with the reasonable first guess that normal people will be basically fine in 1/3 gravity. (This maybe just a question of what we mean by "basically". To me, a 10% lowering of life-expectancy is acceptable and expected when colonizing another planet. The trip itself will probably be a few percent chance of death.)
On the other hand, we have fairly large, well-quantified risks for radiation exposure of long-term living on Mars, and we have no obvious way to solve it without huge concrete structures, underground tunneling, or as-yet-uninvented magnetic shields. EDIT: Actually, I retract my claim that this is a bigger worry than low gravity as I am no longer sure. The radiation exposure is less than I thought, and likely reduces life expectancy by less than 5%. https://www.nasa.gov/sites/default/files/files/1_NAC_HEO_SMD... http://journals.plos.org/plosone/article?id=10.1371/journal.... http://www.space.com/18753-mars-radiation-manned-mission.htm...