add key value pair to existing array

foreach($users as $index => $user) {
    $users[$index]['age'] = getUserAge($user['userid']);
}

foreach($users as &$user){
    $age = getUserAge($user['userid']);
    $user['age'] = $age;
}

Compact Version:

foreach($users as &$user){
    $user['age'] = getUserAge($user['userid']);
}

Note the ampersand before the array variable name meaning the variable is passed by reference, and so can be modified. See the docs for more info.