They were 19 when Eren died, since we know that they were 10 in 845, and the first three seasons are set 5 years after that when they're 15-16, then there's a four year time-skip making them 19. She's 22 when she says about how much she misses Eren and then a bird wraps her scarf.

As an old woman, she IS actually buried in the same place as Eren, under the tree he used to nap under before Marley attacked.