l'm not telling a 16 year old girl with a fragile self-image she is a 4.

So you're not telling a 16 y.old that she's a 4 but you'd have no prob telling a 20 or a 21 y.old they're a 4? What about 18? This is ageist and discriminatory. Always avoid ratings unless if the user has specifically asked for one. Someone can equally have personality or self esteem issues regardless of his/her age. That's the way a correct criticism would give someone very polite but also honest feedback of someone's looks.

Example of a polite but honest criticism: I got a pm from a girl asking if she's ugly and if her nose looked abnormal. My answer: " I don't think you're ugly at all. You seem to have nice features, pretty eyes, flawless skin and seem to be be fit. Also your nose didn't look that big in those pics; actually I wouldn't notice it at all if it wasn't for the 3rd pic (side profile). I don't know if other people in this sub suggested a rhinoplasty but I wouldn't say this to anyone, only if it would make someone feel better

Here's a small tip: maybe you could let your bangs grow? I used to have the same hairstyle in high school and it made my nose look bigger than it was"

Now that's both honest and polite right?