I'll give you the fact that Faber wrote the book in the early 1900s when ƈσmmυɳιsm, feminism, and modernism had solidified into powerful forces into the world and did quite a bit of damage. And as the topic of this thread suggests, the concept of adolescence solidified and probably encouraged parents to slow down the maturation process of their children. So it is possible that he witnessed the young girls at his parish mature slower over the years. I did not read the book and know his specific reasoning.
Be that as it may, I still find the year 20 an ungrounded arbitrary standard. If most girls ended their formal education at ages 15-17 back in 1907, if they were fairly well behaved girls, why did they have to grow 3-5 years in holiness until they were ready. Why not grow 10 years in holiness and wait until they were 25-27? They could still have 5+ children if they marry at 27.
Of course, just because you are ready to get married and God calls you to get married does not mean you will find a spouse within 2 years or even ever. With half the country unmarried, you are not guaranteed a spouse even if you truly desire it and follow the will of God. Telling a 17 year old to wait until 20 may force her to miss a golden once in a lifetime opportunity to follow God's will.
My point is that the Canon Law ruling is not just a moral standard but should also be viewed as a valiant goal to set for parents and the children to be ready at 14 (girls) or (16). There is no reason why anyone's overall human development should be unnecessarily delayed.
Couldn't he take it on a case-by-case basis instead of trying to update the unofficial cultural laws of the church in the United States?
The reason I mentioned St. Therese of Liseux is because she is an extraordinary model of holiness at 15. The religious life is objectively more perfect and thus more difficult. This is not to suggest marriage is imperfect and so much easier. But if she received this dispensation at 15 for a religious vocation, it can be reasonably assessed that a similar model of holiness in girls called to marriage can be reached with another year or two (age 16-17) with proper direction while Fr. Faber seems to suggest 5 years (age 20).