I wonder where you stand on this?
I had been doing some proofreading and wanted the word ‘youngest’ changed to ‘younger.’ If you have 2 sons, then as far as I’m concerned, you have a younger son and an elder son. To use the word youngest or eldest you would need more than 2 sons! That was definitely what I was taught, many years ago.
I was quite surprised to be challenged on this. Inevitably the internet provides a rich seam of information about adjectives, comparatives and superlatives. It appears that late 18th century grammarians decided that the use of a superlative required more than 2 of something. Before this time it was quite acceptable to do otherwise. So, when is a rule a rule and who is allowed to make or change them?
In the end I think usage is all. Language needs to communicate and be clearly understood.
It should still have been ‘younger’ though.