The short answer is that it varies. But while there is no universally accepted standard regarding the age at which people become senior citizens, most Western industrialized nations consider the onset of old age to be at age 60 or 65. That's when most Americans retire and become eligible for assistance programs based on age.

Senior Citizen: This phrase, which can refer to either an elderly, retired individual or someone who's already planning for retirement, typically describes someone at least 60 or 65 years old.

Most developed Western countries set the retirement age around the age of 65; this is also generally considered to mark the transition from middle to old age. Reaching this age is commonly a requirement to become eligible for senior social programs.

Social Security benefits can begin as early as age 62 or as late as age 70. If you have paid into the Social Security system and are eligible for benefits, you can receive a reduced payment when you turn 62. To receive your full benefit, you can wait until you reach your full retirement age, which is usually age 66 or 67.

The answer depends solely on the government agency, establishment, or organization asked. For example, AARP offers senior membership at age 50 while the U.S. government says you can retire at age 62.