Personally the term "young mum" makes me cringe a bit.. Same as "older mum" suppose because they both can be used negatively. I think just mum is fine on its own, lol.
I've known personally mothers at 18/19 to be in secure relationships, have secure jobs and be fantastic mothers despite they're age being below the social norm. I've also known mothers in their late 20s/early 30s who are dreadfully immature and like a child having a child, despite fitting bang into the average age group.
I've also found living in different areas the idea of what is young for a mother varies. I've known some areas where early 20s is the norm to have your first, equally some areas where the norm is mid 30s. Therefore early 20s seems young in comparison.
Made me interested in other people opinions of what is young, as I find it difficult to say myself 