But as the New Year has so far been marked by dramatic volatility in equity markets, at least one proponent of TDFs is concerned fund managers have investors exposed to too much equity risk at the end of their glide paths.

“I think we've forgotten 2008,” says Ron Surz, the president of Target Date Solutions, an Orange County-based consultancy that has patented a target date allocation model.

Surz is a radical in the target-date industry. He doesn't think TDFs should hold any equity risk as investors come to the end of their glide path. He fears 2016 could be the year the chickens come home to roost for TDF investors with a 2015 glide path.

Data from Morningstar says the industry average for equity exposure at the end of target-date glide paths is in the neighborhood of 50 percent. A 20 percent decline in equity markets could mean a 10 percent loss in TDF value for someone retiring this year, presuming half their assets are allocated to equities.

That, of course, presumes fixed-income assets won't be equally volatile.

“Don't get me wrong—I like target date funds for their diversification and risk control capabilities,” says Surz. “But what is the upside to taking risk with life savings at retirement?”

He thinks fund companies have been in a horse race since the credit crisis, eager to capture the upside in equities and incentivized to take more risk than they should.

Money managers get paid more from equities than from fixed-income investments, or for that matter money markets, which is where Surz puts assets at retirement age in the funds he designs and manages.

Some fund families attracted more inflows to their TDF offerings last year than others.

This chart (right) provides a breakdown: