20 CFR 218.41: When a child annuity ends.

A child annuity ends with the earliest of—

(a) The last day of the month before the month in which the child marries;

(b) The last day of the month before the month in which the child dies;

(c) The last day of the month before the month in which the child attains age 18 if the child is not eligible for an annuity as a disabled or student child;

(d) The last day of the last month in which the child is considered a full-time student, as defined in part 216 of this chapter, if the child is a full-time student age 18 through 19; or

(e) The last day of the second month after the month in which the child's disability ends, if the child is over age 18.