Swift version 4.1 (swift-4.1-RELEASE)/usercode/main.swift:5:18: error: immutable value 'self.age' may only be initialized once self.age = 5 ^ /usercode/main.swift:2:9: note: initial value already provided in 'let' declaration let age: Int = 3 ^ /usercode/main.swift:2:5: note: change 'let' to 'var' to make it mutable let age: Int = 3 ^~~ var