Almost 40% of the younger group and 31% of the older group have not met the threshold for the full company match. 

"Automatic enrollment has significantly improved participation in 401(k) plans for all employees over the past 10 years — but even more so for young workers," said Rob Austin, director of retirement research at Aon Hewitt. "However, once they're in the plan, young workers seem to fall victim to inertia with many continuing to save only at the default rate, or slightly above, and risking their long-term savings by not receiving the full employer matching contributions that are offered." 

According to Aon's figures, this can be very costly.

It gave an example of a 25-year-old making $30,000 a year and working for an employer that provides a typical company match: a dollar for every dollar up to 6%. If the employee started off saving the full 6% match amount from Day One and stuck with it until retirement, by age 65 her 401(k) would have more than $950,000 in it. 

However, if that worker waited until age 30 to get to the 6% full match level, she'd have less than $715,000 by age 65, with the delay costing her $225,000. To make that up, she'd have to increase the savings level by 4% and sock away 10% per year for the next 35 years. 

One way to fix that, according to Austin, is for companies to educate employees more thoroughly on the value of the match and encourage them to save enough to get the whole thing.

Another is simply to launch employees on a full-match contribution trajectory upon enrollment into the plan — and to offer automatic contribution escalation to make sure their savings keep increasing over time.