Solution

I made a bunch of assumptions because there wasn't enough information from you question. Please tailor this to your needs. In this example, I've created an array of objects that contains a name and a dob. Since age will be calculated, I've used the html5 field output.

Essentially, what you need to do is create a function that calculates the age. Since you are storing dob, it's unnecessary to store age. Age should be a calculated field based off of dob. Since UTC time is from 1970, calculation of age should be (today - dob).getFullYear() - 1970. See below.

This avoids the multiple id problem that you face. With AngularJS, you should always think data first. Manipulation of the DOM should be a last resort. I suspect you were calculating the age based off of the id's.

Here is the jsFiddle.

Code

Here is my AngularJS Template:

<div ng-app="app" ng-controller="ctrl">
    <div ng-repeat="person in data">
        <input type="text" ng-model="person.name" />
        <input type="date" ng-model="person.dob" />
        <input type="output" value="{{ getAge(person.dob) }}" />
    </div>
</div>

Here is my Controller, along with the helper functions.

var app = angular.module('app', []);

var ctrl = function($scope) {
    /* Replace data with your resource/service call */
    $scope.data = [
        {name: 'Fred',
         dob: randDate()},
        {name: 'Bob',
         dob: randDate()},
        {name: 'Julie',
         dob: randDate()}
        ];

    /* Calculates the age */
    $scope.getAge = function(dob) {
        var now = new Date();
        var age = dateDiff(now, new Date(dob));
        return age.getFullYear() - 1970;
    };
};

function dateDiff(date1, date2) {
    return new Date(Math.abs(date1.getTime() - date2.getTime()));
}

/* Creates a random date in ISO format */
function randDate(){
    var now = new Date();
    var date = new Date(Math.random() * 
                    Date.UTC(now.getFullYear(), 
                             now.getMonth(),
                             now.getDate()));
    return date.toISOString().substring(0, 10);
}