Levi age was never confirmed, however it was rumoured that hes is around the age from 30-33. Eren joined the survey corps when he was 16, and is 19 in the Marley Arc. I am not sure what age Eren is in this Dj since *spoilers*
Erwin is dead, but in this DJ, Erwin is somehow alive and gets to see the ocean. So we are not sure what age Eren is in this DJ. Lets just hope that Eren is old enough. However Levi and Eren never did anything sexual, and its quite normal to be in relationship with large gaps. Im not defending, I just want to get the information out :D
actually, they're age gap is around 15 years, and even if it was 21 years, I know lots of couples who have a 20+ age gap.
This disgusting Levi is 40 and eren is 19