i'm 13 years old, if i lost weight would i like "sag" or become "flabby"

You should be ok. If you are wanting to lose weight exercising regularly is a must but if your worried about "sagging" or becoming "flabby" you can incorporate strength training into your daily exercise routine

No, definitely not.

Negative effects - fat women will no longer find you attractive, and you won't be able to eat doughnuts and drink soda every day.

At your age your skin should still have elasticity so you shouldn't sag. This is a good time for you to lose weight, if you wait too long your skin will lose your elasticity and you will wind up with sagging skin.

only if you are morbidly obese

Depends just how fat you are. That could happen or stretch marks, or both.

no u dont need to worry about that until ur late 20s' ..