Just saw this article courtesy of Greg Mankiw. In the article the authors talk about the impact of rising the drinking age from 18 to 21 in the US. There were two parts – a voluntary lift in some states, and later on a legislated increase for the whole country.
The key bit for me is this (highlighting by me):
The results are striking. Virtually all the life-saving impact of the MLDA21 comes from the few early-adopting states, not from the larger number that resulted from federal pressure. Further, any life-saving effect in those states that first raised the drinking age was only temporary, occurring largely in the first year or two after switching to the MLDA21.
So this isn’t saying that lifting the drinking age is necessarily a bad idea – but that it isn’t necessarily going to save lives either. In the end only the states which did it voluntarily (where it was more of a community effort) had any impact – and even that was only temporary.