Some forget to do it, some just don’t think it’s worth it. But either way, millennials are leaving money on the table.

About three in 10 credit card users (31%) have never redeemed their credit card rewards, according to a Bankrate.com survey of more than 1,000 adults. And the under 30 crowd is the most likely to do so: 34% of people ages 18-29 say they have never redeemed their credit card rewards, compared to just 27% of the 30-49 crowd.

That often means they’re leaving cash, free trips and other perks on the table — a mistake that can cost you hundreds and even thousands of dollars, says Bankrate.com credit card analyst Robin Saks Frankel. Let’s say you get the popular Gold Delta Skymiles card. If you spent $1,000 in the first three months of getting it, you’d earn 30,000 miles. That could get you a free flight to a number of places, or you could use their “Pay with Miles” feature — which lets you get $50 off your airfare for every 5,000 miles you have — to shave $300 off your airfare. But neglect to cash those rewards in, and you’re leaving that $300 on the table.

Even if you do redeem the rewards, but take your sweet time doing it, you’re likely losing out. “Credit card rewards don’t usually gain value over time,” says Frankel. “In fact, they’re more likely to lose value as companies require more points or miles for the same perks. Your best move is to cash them in regularly.”

Frankel thinks many people likely forget about their rewards. Millions of people stay loyal to a single card forever, a CreditCards.com survey from last year found — and thus may have just forgotten what exactly their card offers. Frankel says that other people may just think their rewards aren’t worth that much to them.

Still, plenty of people are all about getting those rewards. “Cardholders typically gravitate to one extreme or the other, redeeming their rewards points/miles frequently (38% did so within the past six months) or not at all,” the survey revealed.

The most popular reward is cash-back: 49% who have used credit card rewards said they most recently exchanged their rewards points for cash. Airline tickets were a distant second (17%), followed by gift cards (12%).