3 young millionaires share the one index fund they’d invest in — plus the early mistake that could cost investors six figures in their retirement funds

• There are over 2,800 index funds and actively managed ETFs to pick from. 
• A less-than-ideal choice could cost an investor six figures over a few decades. 
• A high expense ratio is a cost that is often overlooked by investors. 

People often turn to index funds or exchange-traded funds to simplify investing while getting exposure to diverse assets.