I trend towards not using their age-when-sunk as their age.
...that tends to lead to questions like 'was Hornet only a year or so old mentally/emotionally?'. Or even worse, poor Shinano.
Well, yes, That's why I add the constant. Way I see it, Destroyers are a minimum of 8-12 years old (depending on class), cruisers are older teens/young adults, between 15 and 22 years old (again, depending on class), and capital ships are full adults, with a minimum age of 25 or so.
Partially completed ships such as poor
Shinano are always under 18, though how much depends on the ship.
So, for example, treaty cruiser
San Francisco CA-38 was 16 when ordered, served 12 years, times the .75 constant for her class, making her (mentally/legally) 25 when summoned.
By contrast, Shinano is mentally around 14. Old enough to begin comprehending adult thoughts, but not yet adult. Through service as a kanmusu, she's rapidly growing, but she's still, mentally, a kid.