bfb3a285_SSA_Handbook___211__If_the_worker___
A disabled worker under full retirement age. [If the worker...] 
Would have been fully insured had he or she turned 62 in the month the disability began. The worker also needs 20 credits out of the 40 calendar quarters ending with the quarter that the disability began (20/40). Note: This does not apply in the case of a person disabled because of blindness as described in §507.
SSA Handbook § 211
If the worker...