Elder abuse and neglect is a growing problem across the world, as populations age faster than ever before. By the year 2050, there will be more old people on earth than children for the first time in history, because of rising life spans and falling birth rates. The question is, who will care for them?

The Associated Press spent months reporting across formats on elder abuse and neglect for an occasional series, Old World. Here are a few things to know:

WHO ARE YOU CALLING OLD?

Researchers can't agree on who is an elder, let alone what elder abuse is. Depending on the country and culture, 60 can be the new 50 — or 70, or 80, or 90. And the definition of abuse ranges from physical violence and financial theft to emotional cruelty and even disrespect.

IT STARTS AT HOME

Most of the elderly live with relatives or at home, and most abusers are family members or friends. Researchers estimate at least 4 to 10 percent of older people living at home worldwide are abused, and 4 to 6 percent in high-income countries. That means by the lowest count, 30 million people across the world face abuse and neglect, and likely many more.

THE INVISIBLE OLD

The protection of social networks such as work or school can evaporate with age, and the old can become invisible in the eyes of society. Only a handful of countries legally require the reporting of suspected elder abuse, compared to dozens for child abuse.

In the U.S., which is considered relatively advanced, the government passed the Elder Justice Act in 2010, compared to 1974 for its counterpart on child abuse. No more than two cents of every dollar spent by the federal government on family violence goes to elder abuse. And studies of domestic violence tend not to include victims over the age of 49.

AGEISM BY THE NUMBERS

The extent of elder abuse is hard to pin down, but other numbers fill in the picture. Only one in five older people worldwide has a pension. More than 100 million people fall into poverty each year because of health-care costs, including many elders. And the suicide rate among men over 75 is the highest in the world.

I'VE FALLEN AND I CAN'T GET UP

About a third of people over 65 fall every year, according to the World Health Organization. Such falls can lead to injury and eventual dependence.

WHATEVER YOU SAY, GRANDPA

Even in the few countries where elder abuse is a crime, cases are seldom prosecuted. One reason is that stereotypes cast elders as lousy witnesses and their abuse as tough to prove.