By the very definition of the term MILF, they can be nearly any age. While it's true that the term is usually more /generally/ used to refer to women in their 30's to early 40's, the defining aspect is supposed to be the M, meaning "mother", which doesn't relate to age.
In other words, if she has a child, she can be a MILF.
Cougar, on the other hand, generally refers to women in their 40's or higher who aggressively pursue sexual activity with with partners much younger than themselves.