add key value pair to existing array

I have an the variable $users set to an array that resembles the below

Array(
    [4] => Array(
        [userid] => 4
        [name] => Mike
        [gender] => M
    )

    [5] => Array(
        [userid] => 5
        [name] => Sally
        [gender] => F
    )

    [6] => Array(
        [userid] => 6
        [name] => Steve
        [gender] => M
    )
)

I then have code that loops through this array to call a function to calculate age.

foreach($users as $user){
    $age = getUserAge($user->id);
}

How do I take the variable $age and add it into $users to result with the follow array?

Array(
    [4] => Array(
        [userid] => 4
        [name] => Mike
        [gender] => M
        [age] => 35
    )

    [5] => Array(
        [userid] => 5
        [name] => Sally
        [gender] => F
        [age] => 24
    )

    [6] => Array(
        [userid] => 6
        [name] => Steve
        [gender] => M
        [age] => 32
    )
)