Can the Matron of Honor be single?

The distinction between a Maid of Honor and Matron of Honor fall into two categories: (1) marital status and (2) age.

Marital Status.

Most, but not all, references define the Matron of Honor as a married woman. Those that do not indicate the Matron must be married indicate she is an older, trusted woman.

Age.

With respect to age, a Matron of Honor is generally thought of as older and more mature than a Maid of Honor.

While, strictly speaking, a Matron is a married woman, it is also acceptable to call an older, unmarried woman the Matron of Honor.