I don't think Toms is talking about children. Rather I think he means as some men age (after say puberty, adults here) their hairline slowly recedes, but does so at a rate that he doesn't exhibit typical male pattern baldness. Sometimes these people reach NW5/6/7 but only at a late age (55-60+).
I think a maturing hairline simply means you have male pattern baldness, but it is progressing at such a low rate that one does not need to worry about hair loss treatments. Meaning, they will be fine at least to the age of 40 without finasteride, minoxidil, etc..
So technically, no, a "maturing" hairline isn't its own thing, but rather a term that is loosely used to describe a very slow progressing subset of male pattern baldness. I suppose the converse would be aggressive male pattern baldness wherein a teenager rapidly balds at an early age.