Lucas Daignault likes to glance at his E*Trade account before school or after his shifts at the supermarket. More days than not lately, it shows a sea of red.

Daignault, who just turned 18 years old, is mostly invested in a fund that tracks the S&P 500. The index is off to its worst start to a year in more than five decades, but he tries not to dwell on it. His strategy is to put about $500 a month into his brokerage account, and he has no plans to stop.