I was in High School, so it "young kid "..we dated for a little longer than a year and a half..it angered me at first, but it was at the point where the relationship was shaky, and it was evident that the end was near, tbh..I got over it quickly..I took responsibility, too, since I had stopped putting effort in it..
Also, she ended up dating the guy that she cheated on me with for 2 years, after our breakup..
I've been with my current girlfriend for about 4 years..if she cheated on me, I would obviously be devastated, and I would never speak to her again, tbh..