Is it true that the length of a baby's bottom crack (butt-crack)
relates to gender? I heard that a girl baby has a longer (higher up
the back) crack than a boy. And if this is true, is it also true for
adults?

I have a lot of pediatrician friends.  To their knowledge, there is no
relationship between a baby bottom and gender in the pediatric age,
the newborn nursery, or any other.

In the newborn nursery, doctors do use a system to test the AGE of a
baby in regards to developmental gestation, which is called the
Ballard, which needs to be done within a certain time of birth, but
pediatricians do not routinely measure the lengths of the bottoms.

I don't know the exact science at the bottom of it, but females and
males do deposit fat different. and it This may makes female
butt-cracks look bigger. (just an idea)