Rem wearing a police cosplay
According to Re:Zero series Rem is 18 y.o since arc 3- 4, 19 y.o on Arc 5, and 21 y.o since arc 7 and Subaru Natsuki is 18 y.o since arc 5 , and in arc 7 he is 20 y.o since arc 7.
These images are based on arc 5.