NEW YORK (TheStreet) -- More millennials, Gen Xers and baby boomers are looking for healthy investment returns in health care and large-blend ETFs, a study of portfolio data at TD Ameritrade(AMTD) - Get Report reveals.

"Those over 65 are turning to ETFs for their efficiency, cost, exposure to emerging markets and diversification of their portfolio," said Lule Demmissie, managing director of investment products and retirement at TD Ameritrade. "Millennials have smaller portfolios and less disposable income to invest."

"Over the past 24 months," she said, "TD Ameritrade has seen a 20% increase in accounts with a 50% increase in assets under management [in ETFs] for clients over the age of 55."

"ETFs are a better choice for greater diversification, because mutual funds often carry with them a minimum purchase requirement of $1,000 to $2,500," Demmissie said.

The findings illustrate a shift in investment trends between 2012 and 2014: Clients ages 26 to 35 hold 13.63% of their total assets in ETFs, while clients ages 66 to 75 have 6.11% in ETFs, and clients 76 and older have just 3.73% in ETFs.  

According to company data, all generations have been stepping back from international stock ETFs since 2012, with the largest decreases coming from millennials and retirees ages 66 to 75. "It has to do with geopolitical risk," said Demmissie. An even greater decline was to be found in commodities ETF holdings, and again, the greatest retreats were made by boomers and retirees. "The [commodities] indexes had been on a downtrend, and our clients are moving on par with the market," said Demmissie. "A lot of these commodities all over the world have taken a hit."

By contrast, large-blend ETFs had the largest allocation growth, with their share of portfolios increasing 7% on average across all age groups. Large-blend ETFs invest in large-cap stocks across a number of industries. "The U.S. market continues to be the most popular market among large-blend ETFs, with ETFs that track the S&P 500 being the most popular, along with the Russell 1000, Dow Jones and S&P 100," she said.

TD Ameritrade further found that investors ages 56 and up are more apt to invest in health care-related ETFs than millennials and Gen Xers, because the industry is more relevant to boomers due to their age.  “Boomers are more familiar with these companies as they are taking more medications than millennials,” she said.

Investing in ETFs also make sense for pre-retirees because of their lower fees. “In this interest rate environment, saving on fees is top of mind,” said Demmissie.

According to Morningstar, the expense ratio for ETFs is an average of 0.25% compared to 1.2% for mutual funds. “Boomers are shifting more assets into ETF for greater efficiency and lower cost,” said Demmissie.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the investments mentioned.