I just dont understand why old men pray on teenagers like 12-13 years old?

I know what you mean. I got whistled at by two middle aged creepy guys when I was eleven years old and so on as I got older. It definitely wont stop as you get older. The best thing you can do is ignore it and don't pay attention to them. If you pay attention to them, even though it's in a negative way, they may think you are interested in them and try to hit on you etc. These older men are very unrealistic but yeah ignore them.