In my paper on the topic I reported the recommendation of a protection for astronauts of a minimum 4 meters of regolith (lunar soil).
As correctly pointed out by Larry, the issue is that thinner shielding with aluminum-reach lunar regolith could actually be more harmful than beneficial. Fast-moving energetic particles raining from space and hitting too thin a layer of regolith would generate slower but not stop “secondary emissions” that would then interact more with human tissues such as the blood.
As plastics or water stop the radiation particles with considerably fewer “secondary emissions”, they may provide more protection with considerably less thickness.
How much protection is actually needed? On Earth, the general public should receive less than 0.5 rem/year. For those who work with radiation, the maximum is 5 rem/year.
It turns out that space projects allow for Astronauts to be cooked with a maximum of 50 rem/year. Somehow, this 100-fold increase on what our bodies were evolved to tolerate is not expected to cause much harm.
Perhaps, the very people that suggest that, they should be volunteered for experiments as human guinea pigs.
Sometimes in 2008, the Lunar Reconnaissance Orbiter probe will provide some more information. There is lots to investigate indeed.
For the time being however, we can play it safe.
It is well known that people above a certain age can more reasonably run the risk of exposure to higher radiation doses, if only because they have a higher chance than younger persons of dying of other causes before developing any kind of radiation-induced tumor.
How about selecting “oldies” as Lunar Astronauts then? Given expected life spans, anybody above 70 would do.