How to calculate the age based on NSDate

Many of these answers don’t properly account for leap years and such, best is to use Apple’s methods instead of dividing by constants.

Swift

let birthday: NSDate = ...
let now = Date()
let ageComponents = calendar.dateComponents([.year], from: birthday, to: now)
let age = ageComponents.year

Objective-C

NSDate* birthday = ...;

NSDate* now = [NSDate date];
NSDateComponents* ageComponents = [[NSCalendar currentCalendar] 
                                   components:NSCalendarUnitYear 
                                   fromDate:birthday
                                   toDate:now
                                   options:0];
NSInteger age = [ageComponents year];

I think this is cleaner and more accurate than any of the other answers here.

Edit

Increase accuracy by setting both birthday and now to noon. Here is one way to do that with a Date extension (in Swift)…

/// Returns a new date identical to the receiver except set to precisely noon.
/// Example: let now = Date().atNoon()
func atNoon() -> Date {
    var components = (Calendar.current as NSCalendar).components([.day, .month, .year, .era, .calendar, .timeZone], from: self)
    components.hour = 12
    components.minute = 0
    components.second = 0
    components.nanosecond = 0
    return Calendar.current.date(from: components)!
}