Change the value that is being set in variable’s willSet block

It’s not possible to mutate the value inside willSet. If you implement a willSet observer, it is passed the new property value as a constant parameter.

What about modifying it to use didSet?

var people:[Person] = [Person]()
{
    didSet
    {
        people.sort({ (a:Person, b:Person) -> Bool in
            return a.age > b.age
        })
    }
}

willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.

You can read more about property observers here
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html

You can also write a custom getter and setter like below. But didSet seems more convenient.

var _people = [Person]()

var people: [Person] {
    get {
        return _people
    }
    set(newPeople) {
        _people = newPeople.sorted({ (a:Person, b:Person) -> Bool in
            return a.age > b.age
        })
    }

}