Social Security benefits are based on earnings averaged over most of a worker`s lifetime. Your actual earnings are first adjusted or indexed to account for changes in average wages since the year the earnings were received. The S.S.A. then we calculates your average monthly indexed earnings during the 35 years in which you earned the most. A formula is then applied to these earnings to arrive at your basic benefit, or primary insurance amount (PIA). This is the amount you would receive at your full retirement age. Retirement age for most people is 65. However, beginning with people born in 1938 or later, that age will gradually increase until it reaches 67 for people born after 1959.