The demands of the 21st century may require the military to fundamentally change the way it evaluates, promotes and retains service members, Defense Secretary Ash Carter said Monday.

In describing the "force of the future," Carter suggested breaking with key traditions that define military careers and culture. For example, he suggested promotion boards should give less weight to seniority and place more emphasis on merit by allowing the most talented young people to move up in rank more quickly.

"[We] have to look at ways to promote people based not just on when they joined, and even more on their performance and talent," Carter told an auditorium of high school students at his alma mater, Abington High School, in suburban Philadelphia.

Carter also questioned the custom of making all service members start at the bottom ranks. Instead, he said the military should allow well-trained people to begin military service in the middle of their career and grant them an automatic midcareer rank to reflect their civilian experience and skills.

"The military's rank structure still dates back to when Napoleon was invading Europe 200 years ago. There are some good reasons for that, but for certain specialty jobs, like cybersecurity, we need to be looking at ways to bring in more qualified people, even if they're already in the middle of their career, rather than just starting out," Cater said.

Carter also suggested the military consider a sort of reverse GI Bill that would promise to pay off student loans for recruits. "As college loans get bigger and bigger, for people with certain skills, we need to look at ways to help pay off student loans for people who've already gone to college," Carter said.

The secretary cast the proposed changes as a collective effort to recruit and retain the best and brightest people at a time when the military requires more high-skilled people and will face stiff competition from high-paying private-sector companies. Carter often cites cyberwarfare skills as a prime example.

Carter made his comments just a few weeks after taking office and suggest he will make reforming military personnel policies a key part of his tenure.

Carter also said retaining the best troops will require more flexible career paths. The military services should expand their small-scale pilot programs that allow some career troops to take a "sabbatical," or leave the active-duty force for a few years and use that time for "getting a degree, learning a new skill, or starting a family," Carter said.

"Right now these programs are very small. These programs are good for us and our people, because they help people bring new skills and talents from outside back into the military. So we need to look not only at ways we can improve and expand those programs, but also think about completely new ideas to help our people gain new skills and experiences.

Today's technology may spur changes to the performance evaluation system. "We need to be on the cutting edge of evaluating performance. Your generation's command of technology is beyond what we've ever seen, and we need to take advantage of the kinds of data-intensive technologies that you use every day ... and apply them to help measure and chart how a person is doing in all aspects of their job, and on a day-to-day basis," Carter said.

And social media could "give our people even more flexibility and choice in deciding their next job in the military."

Carter offered few specifics about how these policies might look in practice. Some of them, like changes to promotion boards, would probably require Congress to change current laws. Others, for example, offering career troops the option of taking sabbaticals, can be pursued by individual services.

Carter warned against the insular mindset that can make the military resistant to change. "The Pentagon can be a pretty closed five-sided box, so we need to think outside of it, and we know that," Carter said.

Studies show only about one-third of today's young people are eligible and qualified for military service. Yet today's military needs to bring in about 250,000 people each year to fill its ranks.

Military recruiting benefited from the surge of patriotism following the terrorist attacks of Sept. 11, 2001. But that was temporary.

"As the so-called 9/11 generation begins to leave our ranks, the Defense Department must continue to bring in talented Americans, from your generation and others," he told the high school students.

"To meet all these challenges, the Defense Department has to think hard about how to attract, inspire and excite people like you."