from: http://www.ssa.gov/dibplan/dqualify2.htm

Social Security work credits are based on your total yearly wages or self-employment income. You can earn up to four credits each year.

The amount needed for a credit changes from year to year. In 2011, for example, you earn one credit for each $1,120 of wages or self-employment income. When you've earned $4,480, you've earned your four credits for the year.

Before age 24--You may qualify if you have 6 credits earned in the 3-year period ending when your disability starts.