20 CFR 216.73: Who may be re-entitled to a child's annuity.

If an individual's entitlement to a child's annuity has ended, the individual may be re-entitled if he or she has not married and he or she applies to be re-entitled. The re-entitlement may begin with:

(a) The first month in which the individual is a full-time student if he or she is under age 19, or is age 19 and has not completed requirements for, or received a diploma or certificate from, a secondary school;

(b) The first month the individual is disabled, if the disability began before he or she attained age 22 and continues through the time of application for benefits; or

(c) The first month in which the individual is under a disability that began before the last day of a 7-year period (84 consecutive months) following the month in which the previous child's annuity ended, or the individual was no longer included as a disabled child in a railroad retirement annuity paid under the Social Security Overall Minimum Annuity (see part 229).