Funnily enough, I'm pretty sure this is the second sweet age gap romance work by Takemiya Jin that has this ending. The other one is Steps, if I recall correctly...
Man, I love this. This author's other work, "A Straight Girl Wanders Into A Lesbian Mixer" is one of my favourites, and I remember the struggle of trying to search for the author's name on MangaDex
For a second there, I misread the last panel... Where it says "she grew up!" right after "let's make a baby!". For a second, I thought it said "she grew one"...