From: http://a-penguin-in-redmond.blogspot.com/2014/02/creating-baby-journal-with-tiddlywiki-5.html

(function(){

"use strict";

exports.name = "age";

exports.params = [ { name: "birthDate" }, { name: "currentDate" } ];

exports.run = function(birthDate, currentDate)
{
    var getNormalizedDate = function (date, referenceDate) {
    var numberOfDaysInMonth = (new Date(date.getFullYear(), date.getMonth()+1, 0)).getDate();
    if (numberOfDaysInMonth < referenceDate.getDate())
    {
        return numberOfDaysInMonth;
    }
    else
    {
        return referenceDate.getDate();
    }
}
var getAgeString = function (birthDate, currentDate)
{
    var tmpBirthDate = new Date(birthDate);
    var tmpCurrentDate = new Date(currentDate.getFullYear(), currentDate.getMonth(), currentDate.getDate());
    if (tmpCurrentDate < tmpBirthDate)
    (
        return "not yet born";
    }
    tmpCurrentDate.setHours(12);
    var ageDays = 0;
    while (tmpCurrentDate.getDate() != getNormalizedDate(tmpCurrentDate, tmpBirthDate))
    {
        ageDays++;
        tmpCurrentDate.setDate(tmpCurrentDate.getDate() - 1);
    }
    tmpCurrentDate.setDate(1);
    tmpBirthDate.setDate(1);
    var ageMonths = 0;
    while (tmpCurrentDate.getMonth() != tmpBirthDate.getMonth())
    {
        ageMonths++;
        tmpCurrentDate.setMonth(tmpCurrentDate.getMonth() - 1);
    }
    var ageYears=0;
    while (tmpCurrentDate.getFullYear() != tmpBirthDate.getFullYear())
    {
        ageYears++;
        tmpCurrentDate.setFullYear(tmpCurrentDate.getFullYear() - 1);
    }
    return ageYears + ' year' + (ageYears == 1 ? '' : 's') + ', ' + ageMonths + ' month' + (ageMonths == 1 ? '' : 's') + ' and ' + ageDays + ' day' + (ageDays == 1 ? '' : 's');
  };

    if (typeof(currentDate) == "undefined" || currentDate == "" || isNaN(currentDate))
    {
        currentDate = new Date();
    }
    else
    {
        currentDate = new Date(currentDate.substring(0,4), currentDate.substring(4,6) - 1,  currentDate.substring(6, 8));
    }
    if (typeof(birthDate) == "undefined" || birthDate == "" || isNaN(birthDate))
    {
        birthDate = new Date();
    }
    else
    { 
        birthDate = new Date(birthDate.substring(0,4), birthDate.substring(4,6) - 1, birthDate.substring(6, 8));
    }
    return getAgeString(birthDate, currentDate);
};
})();