Question about age norms of MLU

[Stephanie M. Curenton]

I am currently working a project in which preschoolers told stories using
the book "Frog, Where Are you?"
I have calculated their MLU using CLAN.

Could anyone suggest papers which talk about the age norms for MLU.  For
example, I have 5-year-olds with MLU of about 5.5.  Do most 5-year-olds
have MLU around this range?

I would also be interested in any papers about low-income and
African-American children's MLU.

Thanks for your help!

--steph