Getting the age of a users account

The answer from @Ralf912 (the second ‘here’ in my comment!) is all you need/want.

This little function should help you out:
(please make a backup of functions.php before adding this code snippet in that file.

/**
 * Display Members registration year
 *
 * @author @Ralf912
 * @return date(year)
 * @uses   echo do_shortcode( '[memberSince]' );
 *
 * @version WordPress 4.6.1
 */
add_shortcode( 'memberSince', 'wpse241751_display_year' );
function wpse241751_display_year(){
    $users = get_users();

    foreach( $users as $user ) {
        $udata = get_userdata( $user->ID );
        $registered = $udata->user_registered;
        printf( 'Member since %s<br>', date( 'Y', strtotime( $registered ) ) );
    }
}

You can use now the following shortcode [memberSince] for explample in a template you can add: echo do_shortcode( '[memberSince]' ); this will display(example): Member since 2016

To add also a month you could change in that function date( 'Y', into date( 'M Y',.

Please read also Codex info about: get_userdata and get_users

Last edit/update

There is also a way to display the outcome by adding the code directly in a Template by using the following snippet.

 $users = get_users();
 foreach( $users as $user )
 {
    $udata = get_userdata( $user->ID );
    $registered = $udata->user_registered;
    printf( 'Member since %s<br>', date( 'Y', strtotime( $registered ) ) );
}

Be aware that you have to remove the shortcode from the template and that when using this approach the function is useless and therefore also should be removed from functions.php.

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