The World Health Organization celebrated its birthday Saturday by focusing on aging, including a host of events, research and information under the theme, "Good health adds life to years." Leaders and nonprofit organizations took to Twitter to mark World Health Day and toast the WHO, whose constitution was enacted April 7, 1948, to affirm its mission as the "directing and coordinating authority for health within the United Nations system." These groups included the Kenyan Red Cross, Grand Challenges Canada, the Cleveland Clinic and the International Diabetes Federation, among others. The U.N. itself also chimed in, with Secretary-General Ban Ki-moon urging civic and private leaders to devote "attention and resources to ensuring that people everywhere have the chance to grow older in good health." That includes realizing the many ways older people contribute to society, as well as knowing what it will take to take care of them as they age. By 2050, the WHO estimates that there will be almost 400 million people age 80 and older -- compared to about 14 million people in that age group around 1950. "Older people make many valuable contributions to society -- as family members, as active participants in the workforce and as volunteers within communities. The wisdom they have gained throughout their lives makes them a unique resource for society," Ban said in a press release. "But more older people also means an increased demand on health care and social security systems." The WHO itself marked World Health Day with festivities at its headquarters in Geneva, Switzerland. The organization's campaign aims, in part, to challenge stereotypes about older people -- like that they can't take care of themselves, can't work or play as well as others, and are no longer attractive. The WHO said it is important that older people are engaged in their societies and in their health, and that citizens and governments are similarly committed. Having sound preparations and actions to address the aging population is not only a moral imperative, but also one that makes sense economically and politically, according to the agency. "If we can ensure that people are living healthier as well as longer lives, the opportunities will be greater and the costs to society less," WHO Director-General Margaret Chan said in the foreword of the "Global Brief for World Health Day 2012." "This great demographic challenge of the first half of the 21st century therefore demands a public health response, and WHO has identified this as a priority."