Total assets under management, the study found, as of December 31 had increased by 4 percent to $227 billion, with net new flows of about $10 billion, while fees went the other way, decreasing to an asset-weighted average of 0.74 percent in 2015 from 0.79 percent in 2014.

The drive by investors to find the lowest costs for both plans and investments has seen a migration toward direct-sold plans, since the typical participant for such a plan pays 20 basis points, on average, in addition to the cost of the underlying funds.

For an advisor-sold plan, the typical participant pays an additional 70 basis points. Direct-sold plans now represent 53 percent of the industry's assets.

Plans with age-based options that have a higher average Morningstar Rating (a quantitative risk-adjusted performance measure) generally have more competitive fee structures or stronger underlying strategies.

The report said that, among both direct- and advisor-sold 529 plans, more aggressive age-based tracks have delivered the best total returns and more conservative tracks have lagged during the past six years, as a result of the recent equity bull market.

The teams handling asset allocation for 529 plans' age-based options have varying levels of experience.

Some plans leverage target-date retirement managers; the report said that the target-date retirement teams from American Funds, Fidelity, J.P. Morgan, T. Rowe Price, and Vanguard collectively managed nearly 50 percent of 529 plan assets as of December 31. Other plans, it found, use less-experienced asset allocators.

Age-based 529 plan portfolios tend to avoid risk and invest less in less common asset classes, such as high-yield bonds and commodities, compared with target-date funds.