"The concept of retirement is evolving," said Christine Russell, senior manager of retirement at TD Ameritrade.

Because of longer life spans, Americans are also boosting their savings to preserve their nest eggs, the TD Ameritrade study showed, which surveyed 2,000 adults between 40 to 79.

One in three Americans who are at least 40 have or plan to have a job in retirement to prepare for a longer life, according to a survey conducted by Harris Poll for TD Ameritrade.