What's the title for addressing an unknown young person the same age as you?
In my opinion, if you are adult and the person who droppd wallet is:


younger than 11, you can call him or her "小朋友";
at the age of 11 to 18, you can call him or her "同学";
at the age of 18 to 24, you can call him "同学", "帅哥(cool man)", and call her "同学", "美女(beautiful girl)";
at the age of 24 to 35, you can call him "帅哥", and call her "美女";
older than 30, it really depend on your age and the person's age. For example, the person is 42, and you are 23, you can call him "叔叔(uncle)", and call her "阿姨(aunt)".


However, you can call the person who dressed very fashionably "帅哥","美女", eventhough he(she) is older than 35.

In addition, I think the person will usually stop walking, if you shout "喂，你的钱包掉了！(Hey, You dropped your wallet.)"