By Hal M. Bundrick NEW YORK ( MainStreet)--Set a goal year for retirement and choose your fund: target-date mutual funds are gaining favor, and investor dollars. TDFs grew to over $500 billion in assets in the first quarter of 2013, and while growth has slowed, fees continue to fall. The asset-weighted average expense ratio dropped to 0.91% in 2012, down from 0.99% in 2011, according to Morningstar. The funds have gained popularity in recent years, especially in 401(k) plans, as the allocation of the investments inside the fund adjust to a retirement goal date. Lingering questions regarding performance are being answered, as the funds have performed within broad market trends since 2008.