How old was Mercedes Thompson when she came to Aspen Creek?

Mercy was put under Bran's care and then the care of her foster parents when she was around three
months old.

Quotes:

"Then they tried to get her to put me up for adoption, but she was determined to raise me herself-until I was three months old, and she found a coyote pup in my crib."
Mercy in Moon Called Chapter 3

"Your mother put you in my arms when you were less than three months old."
Bran in Silence Fallen Chapter 14