It should be obvious, given the relatively short track record of the strategy, but Ibbotson has found that asset allocation in target date funds has shifted dramatically over time.
The Ibbotson analysts discovered that the "system" by which managers adjust their portfolios as the target date approaches is often still a work in progress.
As a result, a 2015 target fund, for example, might finish its glide path with a very different asset class mix compared to the target date funds that those retiring in 2000 or even 2010 might have bought.
This is even true within fund families. Fidelity, notably, has raised its terminal equity allocation from 30% to about 50% over the last 10 years, while Vanguard has made a similar move since 2006.
Because of this, Ibbotson confesses that it's practically impossible to tell how tomorrow's retirees will be invested, which makes planning difficult at best.