Aaron was older than Moses and was not threatened by the decree of Pharaoh which took place at a later date. It only concerned those being born after the decree.
Ex. 2:1-2. Notice of Moses’ birth is commented on briefly. Obviously Pharaoh’s decree (1:16, 22) jeopardized Moses’ life. The names of Moses’ parents are not given here but in 6:20 it is learned that his father was Amram and his mother Jochebed, Amram’s aunt. This Levite couple had two other children: Miriam (15:20) and Aaron (6:20). Aaron was three years older than Moses (7:7). Assuming that the Exodus occurred in 1446 and since Moses was 80 years old at that time (7:7), he was born in 1526 b.c. at the beginning of the reign of Thutmose I (1526-1512) or at the end of Amenhotep I’s reign (1545-1526).