They've been trying to change the minimum age to 18 for a while now, not that anyone ever gets married at 16 or 17.
A demographic for it seems to exist though, but it's actually more of a hot teacher fantasy kinda thing. There seems to be at least 3 different manga where the main theme is "I'm a 16-17 year old married to her super hot teacher".
How many kids do you want?
I want seven♥Newly married!When relaxed, speaks while caressing her tail.Naofumi-kuunNaofumi-kun, good morning!Ohh...I've got a crush on Naofumi.Everything♥The blazer looks good on you.What part of Iwatani is good?JK Raphtalia-sanJK - joshi kousei - highschool girl