It's rough, but at some point there needs to be an arbitrary line.

I think 21 is used is many cases because that's supposedly when the brain has completed most of it's growing or something about pre-frontal cortex being fully developed.

In fact, the Catholic faith identifies seven years old as the "age of reason" or accountability: the age at which a child can be held morally culpable, tell right from wrong, and is eligible to receive Holy Communion and therefore, the Sacrament of Penance as well.

https://www.newadvent.org/cathen/01209a.htm

However, we also have a legal concept of "age of majority" which was 21 until 1983. https://en.wikipedia.org/wiki/Person_(Catholic_canon_law)#Ag...

I would explain that the wide gap of 11 years where a child is morally but not legally culpable, is due to the fact that the child is still a child, and in the care of his/her parents, and the parents should ultimately be legally responsible for the child's actions until the child is demonstrably independent of their household and authority.

It is clear that Islam and Judaism take puberty into account. It is not clear to me, historically, whether Christianity ever used puberty as a criterion. It is important to note that "age of consent" is not the same legal concept as "marriageable age".