They also have the biggest mortgages relative to their incomes and their average unsecured debts are almost a third higher than the national average at £5,863, according to Alliance & Leicester's latest Borrowing Monitor research.
Not only is their debt level the highest, but it is likely to stay that way for several years.
The report shows they are the group most likely to only make the minimum credit card repayment, and the least likely to pay in full.
Overdrafts are popular, with those in their early 30s often keeping it as a rolling, if expensive, flexible loan facility. More than other groups, they are also have a personal loan and among the most likely to miss their monthly payments.
Contrary to the stereotype, people in the "Club 18-30" generation are less likely to have consumer debts than their older counterparts and when they do, they have lower balances than most older groups.
However, they are usually weighed down by student loans, which account for 47% of their overall borrowings, by far the largest type of debt they have.
Chris Rhodes, director of retail banking at Alliance & Leicester said high debt levels were hitting people in their early 30s because at that stage of life they began to settle down with a partner and buy their first home. Two thirds (66%) live with a partner and 63% are homeowners.
He said: "The early 30s are a transitional age where careers are taking off and before family responsibilities kick in. Many are buying their first homes at this point, but are also enjoying rapidly rising salaries and are keen to enjoy life to the full.
"Some, particularly those trying to get on the housing ladder, may find themselves in financial difficulty as a result of living beyond their means.
"The picture for the under-30s is dominated by student loans. A hangover of student debt is constraining their appetite for other borrowing and delaying their ability to get on the housing ladder."
