Investing.com - Once considered stodgy, target-date mutual funds had their best year ever in 2017, attracting $70 billion in new investment, according to a report by Morningstar Research. Target date funds now have $1.11 trillion in assets under management, six times more than a decade ago.The funds, also known as life-cycle funds, are designed to grow assets over a pre-determined period of time for a specific goal, and are popular with company retirement plans.A key driver of the growth has been the proliferation of passive target date funds, ones that invest in low-cost index funds.Management fees have fallen by about a third over the past five years to an average of 0.66%.