I just want to add some information to the accepted answer as I believe it is imprecise.
小x and 老x are used directly with someone you are familiar with (or speak with regularly, like a colleague).
The usage is based not only on age difference but also on the social status, gender, family relation to you and who is your interlocutor.
(You will not call your boss 老王 just because he is older than you but you would call him 老王 when talking about him with some colleagues. Similarly you could call 老张 an 80 years old chess partner that you play often with - there is no age limit when using 老. It's a mark of respect, not a mark of age like "old" in English.)
There are many ways to talk to/about someone:
-Using a diminutive like 小x and 老x
Both terms indicate that you converse or interact with the person on a regular basis but not regularly enough to be very good friends, intimate or family.
- boss talking to an employee (小)
- employee talking about the boss to a friend (老)
- young chess club player talking to older friendly player (老)
- 25 years old employee talking to 25 years old trainee (小)
-Using the full name
- Old person
-Using the first name
Rare. Can be used in the same place were you would call someone 小x
-Using a nickname
Obviously if you are close enough to this person.
-Using his/her specialty
-Adding Mr or Miss or Mrs
If you don't know how to call someone, use those, you cannot be wrong.
-Using the family relationship
Usually uncle/auntie and brother/sister (who are not your real brother or sister)
- My wife's uncle: 王叔叔 or 叔叔
- My wife's auntie: 王阿姨 or 阿姨
- 5 years old talking to a 20 years old girl: 姐姐 or 大姐
- 5 years old talking to a 20 years old boy: 哥哥 or 大哥
- 5 years old talking to a 50 years old woman: 阿姨
-First name + family relation
- My wife's relative (male), a little older than me: (王志明) 志明哥
In your case (uncle Ju) you can definitely call him (or talk about him) 居叔叔 or 叔叔
I would avoid the 老/小 diminutives when talking to/about anyone for the moment until you are familiar with the nuances or until someone asks you to use it.
The list above is of course subject to exceptions and difference of usages in certain provinces but in general it works.