型推論いいじゃん
コーディング量が減るし

let age: Int = calculateAge(fromBirthday: birthday)
let age = calculateAge(fromBirthday: birthday)

let name: String = nameField.text.stringByTrimmingCharactersInSet(.whitespaceCharacterSet())
let name = nameField.text.stringByTrimmingCharactersInSet(.whitespaceCharacterSet())

紛らわしい変数名とか使わなければ型は自明なケースも多い