- Is the gradual loss of hearing in both ears. It's a common problem linked to aging. - Are abnormal accumulations of a protein called tau that collect inside neurons. - Is a general term for loss of memory, language, problem-solving and other thinking abilities that are severe enough to interfere with daily life. - Comparing population pyramids over time can explain a lot about what life is like in a place, as well as its economic and social challenges, important data visualization tools for forecasting social and economic change.