Aged Content Message

It’s right there, under General Settings in your admin menu. Configurable settings are:

• Activate Message: By default, no message will show up on your website until you activate it here.
• Minimal Post Age: Set a number of years for posts to be considered aged. Default: 1.
• Message Heading: Heading text. Default: The times, they are a-changin’. (Dylan, google him.)
• Message Body (Singular): Singular form of the message text, for 1 year-old posts.
• Message Body (Plural): Plural form of the message text, for 2+ year-old posts.
• Message Class Attribute: CSS class name(s) applied upon the message’s wrapping <div>.
• Message HTML: HTML template for the message. You can completely control its output via this field if you want, or use placeholders for heading and text.
• Message CSS: Customize the visual styling of your message right here. Or not.

Sure, just empty the CSS field and no styles shall be applied. If you want to add styles to your theme instead, this might get you started:

/* Default styles */
.aged-content-message {
    background: #f7f7f7;
    border-left: 5px solid #f39c12;
    font-family: inherit;
    font-size: .875rem;
    line-height: 1.5;
    margin: 1.5rem 0;
    padding: 1.5rem;
}
.aged-content-message h5 {
    font-family: inherit;
    font-size: .8125rem;
    font-weight: bold;
    line-height: 2;
    margin: 0;
    padding: 0;
    text-transform: uppercase;
}
.aged-content-message p {
    margin: 0;
    padding: 0;
}

You can still use those good old filters from v1.3, like for this conditional handbrake that doesn’t have a setting (yet):

/* Set condition for displaying message to include pages. */
function yourprefix_aged_content_message__the_content_condition() {
    return ! is_single() && ! is_page();
}
add_action( 'aged_content_message__the_content_condition',
    'yourprefix_aged_content_message__the_content_condition' );