Newly installed Armed Forces of the Philippines chief of staff Lt. Gen. Gregorio Pio Catapang Jr., left, next to Philippine President Benigno Aquino during Lt. Gen. Catapang's Change of Command ceremony on July 18. Reuters

MANILA—The newly appointed chief of the Armed Forces of the Philippines must juggle counterinsurgency campaigns against communist and Islamist rebel groups, a sweeping modernization program and worsening territorial disputes with China.

But there is one obstacle Lt. Gen. Gregorio Pio Catapang, who became armed forces commander last week, won't overcome: early retirement.

Like all recent Philippine commanders, Gen Catapang, 55 years old, is constrained by a policy that forces military chiefs to retire at 56, often within months of taking the reins.

The result of the age cap is constant turnover. Gen Catapang becomes the 17th armed-forces chief since 2000.

Recent commanders have run the military for an average of just 10 months, and none has completed two full years in the top job. It isn't possible to appoint younger people since leading candidates are usually in their mid-50s by the time they become three-star generals.

Lt. Gen. Gregorio Pio Catapang, Jr. assumed command on July 18. Rouelle Umali/Xinhua/Zuma Wire

While the role of chief of staff is akin to the chairman of the Joint Chiefs of Staff in the U.S., with authority over the Philippine army, navy and air force, the U.S. has appointed only four individuals to its top military post during the same period. The U.S. government hands chairmen two-year terms, but they usually are reappointed and serve four years in total.

Critics of the high rate of turnover in the Philippines say it undermines the new commander's ability to get things done at a critical juncture for the Philippine military.

"What can you do working for less than 365 days?Nothing really," said Clarita Carlos, a politics professor at the University of the Philippines and a former president of the National Defense College of the Philippines. "You're just warming the seat."

Spurred by its sovereignty disputes with China in the South China Sea, the Philippines is embarking on an ambitious program to expand and modernize its armed forces, efforts Gen. Catapang must now oversee.

Last year, President Benigno Aquino III said he would add 20,000 troops to the military's 124,000 force over the next three years and unveiled plans to spend $1.8 billion on weapons to reverse decades of low investment in the military.

Naval frigates and fighter aircraft are among the systems the military is now acquiring to address its lack of air-combat capability and restock its outmoded naval fleet.

The AFP is also engaged in the delicate process of ushering the U.S. military back into the Philippines after a 20-year absence, following the signing of a new defense pact in April that will see U.S. forces deploy to a maximum of five Philippine military facilities on a rotational basis.

On top of these new missions is the decades-old task of waging conflicts against the numerous militant groups that operate across large areas of the country.

Gen Catapang turns 56 on July 11, 2015; he was unavailable for an interview. With less than a year to make an impact, he has been given an impossible remit, Ms. Carlos, the politics professor, suggested. Even so, there is little appetite to fix the defects in the military appointments system, she said.

"Our military has a structure carried over from the American occupation," said Ms. Carlos. "There is a deeply entrenched bureaucracy there."

Ms. Carlos said the policy of appointing AFP chiefs with only months left before mandatory retirement was instituted by Gloria Macapagal Arroyo, Mr. Aquino's predecessor, to curb the political influence of the military and reduce the likelihood of a coup d'état.

Upon taking office in 2010, Mr. Aquino pledged to remove the mandatory-retirement rule, but he has shied away from securing longer tenures for the top brass. In 2012, he vetoed a bill that would have introduced a fixed, three-year term for military chiefs, arguing that it would be unconstitutional to let soldiers serve beyond the mandatory retirement age.

The constitution prohibits the extension of military service beyond the age limit set by law, though it doesn't state that the retirement age must be 56.

As a result, what is referred to in the Philippines as "the revolving door" has kept on turning: Gen. Catapang is Mr. Aquino's fifth AFP chief as his presidency enters its fifth year.

Ms. Carlos said that deeply ingrained resistance to change and vested interests within the military establishment, rather than the constitution, were the real barriers to modernization.

AFP spokesman Lt. Col. Ramon Zagala said the military had no choice but to discharge its most senior commanders once they reached the age limit. "That's the law, and yes, it's a big challenge for us," he said. "Definitely that law must be looked at again."