No, not really; the mid 40s is middle age. If someone is still acting like a "kid" at that age, then they are a case of arrested development.
I understand all that pretty well, my daughter, who is in her early 20's, might be a kid, but she's certainly more mentally grown up than plenty of folks in their 50's that I know. I was merely making a wan remark about how at my age, someone in their mid forties, like Miss Coulter, looks very young.