How can I add Variables into a String?(Swift)

In swift, string interpolation is done using \() within strings. Like so:

let x = 10
let string = "x equals \(x) and you can also put expressions here \(5*2)"

so for your example, do:

var age:Int=1
var pet:String="dog"
lblOutput.text = "Your \(pet) is \(age) years old!"