Here is code I recently used to calculate persons age in Years, Months, and Days on Arduino:

 
#include "RTClib.h"
RTC_DS3231 rtc; // Using DS3231 Real Time Clock 
DateTime now = rtc.now(); // Current Date and Time
DateTime dob = DateTime(1984,8,16,0,0,0); // Date of Birth (1984.08.16 00:00:00)
 
int t0 = dob.year() * 12 + dob.month() - 1;
int t = now.year() * 12 + now.month() - 1;
int dm = t - t0;
int Y;
int M;
int D;
 
//Calculate age in Y/M/D
if(now.day() >= dob.day()){
  Y = floor(dm/12);
  M = dm % 12;
  D = now.day() - dob.day();
}
else {
  dm--;
  t--;
  Y = floor(dm/12);
  M = dm % 12;
  DateTime tmp = DateTime(floor(t/12),(t%12)+1,dob.day(),0,0,0);
  D = (now.unixtime() - tmp.unixtime())/60/60/24;
}
 

Credit: I found this algorithm on stackoverflow.com and modified to work on Arduino.

April 2017