People in mid 30s How do you address strangers younger than you ?

So, I am a male in mid 30s and this weekend I was in a nearby shop to get groceries, there was a girl probably in early 20s (I am very bad at guessing age) she was standing near a shelf where I wanted to pick something. I waited and then said excuse me she didn't move after that I said "bete, excuse me" and she turned and looked at me like I have said something very very wrong. Not to say I got terrified with that look and ran picking my besan packet. I am sure she heard the word "bete" because the helper standing there laughed and said "didi ko chota bacha bana dia apne" After this of course I just quickly went to the counter and paid. I noticed she was again giving the same look when I left. My question is how do you address people younger than you at this age. I have been quite conscious of how I address strangers. So thought of asking this sub for suggestions.