Growing older usually means getting more disciplined financially. Some millennials say they aren't quite there yet.

Nearly 1 in 3 millennials say they have spent money budgeted for other things on themselves, according to a new report from Northwestern Mutual. They're more than twice as likely to do so than Gen Xers, and almost eight times more likely than baby boomers.

The new analysis is part of Northwestern Mutual's 2017 Planning & Progress Study, which surveyed almost 3,000 American adults in mid-February.

Millennials also admitted to two other big money sins. One-third said they were prone to excessive or frivolous spending, and almost a quarter said they have hidden purchases from a spouse or partner — a higher rate than either Gen Xers or boomers.

Tendencies toward unexpected spending make it all the more important to have a structure in place to monitor your budget and cash flow, said Samuel Boyd, a certified financial planner and a senior vice president at Washington-based Capital Asset Management Group. Leveraging technology to help you automate the process is useful, too.

"[That] really helps people stay within realistic spending patterns," he said.

Boyd said he favors the 50-20-30 budget: Putting 50 percent of your income toward essentials like housing, groceries and debt payments, 20 percent to savings and 30 percent to discretionary purchases and other "wants."

Having a sense of where your money is going means you don't have to feel as bad about spending a little extra on wants one month, Boyd said. You can more easily find other places to make up that gap.

Moving away from technical advice and seeing the value of perspective is one way certified financial planner Bill Simonet helps millennials focus on the long-term and cut out unnecessary expenditures.