how to add a whitespace in PHP in input box?

I was trying to print default value of age in a input box with php

<input type="text" class="form-control" placeholder="age *" value=<?php  echo $_SESSION['age'].."year";  ?> />

like $_SESSION['age'] value is 21 , I expecting text in input box like "21 year" but it gives me "21year"

I want one whitespace between 21 and year. please help me ....