User inputs DOB with dropdowns for month and date, with a text box for year. Since month is 'January, February, etc.' and we need its numerical value, we included a switch statement to convert it to a number. We then add those together into 'birthday', pass that value to CurrentAge(), which returns the user's age as YEAR-MO-DY.

age.js

        $('#birthdayBtn').on('click', function(){
            var month = $('#birth_month').val();
            switch (month){
                case 'January':
                    month = 1;
                    break;
                case 'February':
                    month = 2;
                    break;
                case 'March':
                    month = 3;
                    break;
                case 'April':
                    month = 4;
                    break;
                case 'May':
                    month = 5;
                    break;
                case 'June':
                    month = 6;
                    break;
                case 'July':
                    month = 7;
                    break;
                case 'August':
                    month = 8;
                    break;
                case 'September':
                    month = 9;
                    break;
                case 'October':
                    month = 10;
                    break;
                case 'November':
                    month = 11;
                    break;
                case 'December':
                    month = 12;
                    break;
            }
            var day = $('#birth_day').val();
            var year = $('#birth_year').val();
            var birthday = (year + "-" + month + "-" + day);
            var current_age = CurrentAge(birthday);
            alert(_age);
        });

    }
};


function CurrentAge(birthday){
    var dob = new Date(birthday);
    var today = new Date();
    _age = Math.floor((today-dob) / (365.25 * 24 * 60 * 60 * 1000));
    return _age;
}

birthday.jade

#pageBirthday.page
	.content
		br
		h4.centeredText What is your date of birth?
		br
		h4 Month
		select.form-control#birth_month
			option Please choose a month...
			option January
			option February
			option March
			option April
			option May
			option June
			option July
			option August
			option September
			option October
			option November
			option December
		h4 Day
		select.form-control#birth_day
			option Please choose a date...
			option 1
			option 2
			option 3
			option 4
			option 5
			option 6
			option 7
			option 8
			option 9
			option 10
			option 11
			option 12
			option 13
			option 14
			option 15
			option 16
			option 17
			option 18
			option 19
			option 20
			option 21
			option 22
			option 23
			option 24
			option 25
			option 26
			option 27
			option 28
			option 29
			option 30
			option 31
		h4 Year
		input(type='text', id='birth_year', class='form-control', placeholder='Year')

		button.btn.btn-default.form-control.button.trackit#birthdayBtn(type='button') Submit