"小王" 和 "老王" how to determine which to use?
Just as @Flake said:


  In general, "小X" should be used to a person who is younger than you while "老X" to an older person.


And here is some additional usages:


Generally speaking, 老X is often used to address a male, and 小X can be used for both male &amp; female.
老X can be used to a person who is an acquaintance to you(usually both of you have almost the same age), for example, a close friend at school. However, in some cases, close friends will use 老X as nickname without obvious distinction between ages(this rarely occurs, but exists).
You should not call a person who is much older than you 老X, it's impolite. You can call him/her X哥/X姐, and if they are older still X叔/X姨, etc.
In companies, superiors might call their subordinates 小X.
And in the family, elder ones might call the younger 小X.


Your uncle is older than both of you, so I think you'd better call him X哥(or just the given name, but should not use 小X as his parents used) when talking to others. :)