"I urge governments, civil society and the private sector to commit attention and resources to ensuring that people everywhere have the chance to grow older in good health," Secretary-General Ban Ki-moon said in a statement.
World Health Day is celebrated annually to mark the founding of the World Health Organization in 1948, and this year's theme for the U.N. health agency is promoting good health to help people remain healthy and productive in later years.
WHO says in the middle of the 20th century, there had been 14 million people in the world age 80 or older but by 2050, that number will increase to nearly 400 million, with 100 million of them in China alone. Within the next five years, the population of people 65 and older will outnumber those under 5 for the first time eve.
The aging population has strained countries' health services, Ban said.
He said the greatest health threats for the elderly are non-communicable diseases, including heart disease and strokes, and the biggest causes of disability are visual impairment and dementia.
The incidence of non-communicable diseases in low-income countries is two to three times greater than in high-income countries, Ban said.
Margaret Chan, WHO's director general, noted simple, low-cost ways to help keep elderly people healthy. She pointed to exercise and affordable medicine to control hypertension.