Click on charts to enlarge, courtesy of Societe Generale.
However, this seems rather interesting:
An obvious criticism of this type of reasoning is that you do not have to be American to buy US equities. Populations are getting younger in a number of emerging markets and some of these foreign savings could be easily be recycled in the US equity market. While not unreasonable, we think that this line of defence is a fragile one. For example, the Japanese experience of the 1990s shows that a demographically healthy West lent absolutely no support to local equity valuations. In addition, even though foreign ownership of the US equity market has been increasing, the US market has still been on a de-rating trend for about a decade.
Japanese have all sorts of experience, also today in FX markets ...