Usually when someone had a Stand they learn about it at the age of ten, so yes. However, they won't know they both have Stands until they try to fight each other with them.
BTW, when a Stand uses an attack, the character usually does the cliche "say the name of the attack" thing. :lol: So if Divide By Zero were to use his attack, Kian would yell "Divide By Zero: Dimension Breaker!!!"