Two long-lost friends Rakhi and Rekha bumped into each other at the supermarket one evening. While speaking about their families, Rakhi, who was fond of puzzling people, mentioned that she had three daughters, Kasturi, Kashmira and Kashish, all of whom were aged below 10 years and had different ages. "When we add their ages together, we get a prime number. The product of the ages of Kashmira and Kashish is equal to that of my eldest daughter, Kasturi." Rekha immediately said excitedly, "I just realised that my son, Karan, is two years older to Kasturi. When they are old enough let's get them married. What say? We'll be in-laws!!!"
How old is Karan? Provide a detailed explanation along with your answer.