What's the title for addressing an unknown young person the same age as you? For example, you are a 20 year old male/female. You are walking on the street, then you see a male stranger who seems to be the same age as your. He is in hurry, you see him drop his wallet. You pick up the wallet and want to give back the wallet to the person. He is walking quite fast and the street is quite crowded, so there is no way to approach him and pat him saying "对不起，你的钱包掉了" (excuse me, you dropped your wallet). You don't want to let him out of your sight, so you decide to shout to him "sir! you dropped your wallet!". How to say that in Chinese?

Can we use 同伴 or 同学 to address him? (eg: 同伴！你的钱包掉了！)
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.)"