If “half your age, plus seven” gets you the youngest age society will accept, then, “minus seven, times 2” should get you the oldest acceptable age.
So (18-7)*2
is 22. A mom with a 3 year-old is still a mom.
And if that doesn’t suspend your disbelief, you could still slap on “step” to the relationship label and the script writes itself.
I didn’t choose the Skux life