Many people underestimate how much money they will need, because spending on leisure activities, such as travel, often increases immediately after retirement, said Douglas Kobak, a certified financial planner and senior advisor and principal with Main Line Group Wealth Management.
For those who retire in their 60s, spending on leisure activities tends to trail off after a decade or so, but that is when health-care costs generally pick up, he said.
Read MoreRetirement years not so golden
Passage of the Affordable Care Act in 2010 actually made it more feasible for Americans to retire early, particularly because insurers can no longer charge higher rates to those with preexisting conditions, and those with low taxable incomes may qualify for subsidies, regardless of their net worth, Carter said.
Still, those who retire in their 40s will have to wait decades, until at least 62, before they are eligible for Social Security benefits, and they are likely to collect less in benefits than Americans who pay into the system for a longer time. They may also need their money to last for 40 to 50 years.