Is -2.75 bad eyesight for a 14 year old?
Put your glasses on and take a visual acuity test.
As DJC stated, there is no standard for prescription strength according to age, or any other factor. As they say now, it simply is what it is. No doubt your vision is 20/20 with your glasses, just like any of your friends with a different prescription. You are all equal when wearing glasses. Worrying about a prescription number is a waste of time and energy. Your slightly stronger prescription only means your eyeball is SLIGHTLY more oblong than someone else's with a weaker prescription. It is NOTHING to worry about.
There's no such thing as age-appropriateness when it comes to eyesight. All different people of all different ages, genders, races, and heights have different eyesight. There are people younger than 2 who are totally blind, and there are people older than 90 who have never worn glasses in their entire life and never will. Eyesight doesn't know age.
In 10 years you can have your eyes corrected by Lasik. Your correction is moderate.
It is a moderate amount of correction. Medium.
-2.75 is not terrible. In my opinion, anything worse than -3 is bad and anything worse than -6 is terrible.