Sometimes getting wrong age

I have created a function as given below that calculates the age in years, month and days based on supplied date of birth. I have noticed that for some dates I do not get the correct results. Not able to point out why this is happening

Code:

NSDate *startDate = [NSDate dateWithNaturalLanguageString:dateofbirth locale:[NSLocale currentLocale] ];
	NSDate *endDate =  [NSDate date] ;
	
	NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
	
	NSUInteger unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit;
	
	NSDateComponents *components = [gregorian components:unitFlags
												fromDate:startDate
												  toDate:endDate options:0];
	NSInteger years = [components year];
	NSInteger months = [components month];
	NSInteger days = [components day];	
	NSString* yearsPassed = [NSString stringWithFormat:@"%dy", years];
	NSString* monthsPassed = [NSString stringWithFormat:@"%dm", months];
	NSString* daysPassed = [NSString stringWithFormat:@"%dd", days];

The below given link will be helpful in cross checking the age calculated by the above function
AGE
A sample date: 14th January, 1959. The above code returns
49years, 11 months and 18 days
But the calculated age from the link [which is correct] is
50years, 4 months and 12 days
I have also tried replacing 0 with NSWrapCalendarComponents in the options but still the same discrepancy
Can anybody point out the reason for this?
Thanks
Arnieterm