I'm not sure you know what canon means, which is the claim I've suggested is wrong.
If you don't do anything, they're 18, ergo canon = 18.
And as I've already said, if they were worried about deniability the 'evil modder' could change the default age in the patch - they do not.
Happily for you, the game allows a range between 12 to 22 to cater for a variety of preferences. 12 to 22 is not <18, a range is not canon.
At best you could argue there is no canon, because the game allows a range. Except the game sets a default of 18 within this range, so there is a canon age of 18. Unless the devs publicly say something which contradicts their own code, which they have not afaik.
I will say you are both mistaken, because of facts.