This code snippet restricts registration for under 18 years old using the custom validation applied to the Birth Date field. You can add this code to the functions.php file in the active theme directory.

um_custom_validate_birth_date.php

<?php

/**
 * Restrict registration for under 18 years old.
 *
 * @param array $args Form Arguments.
 */
function um_custom_validate_birth_date( $args ) {
	if ( ! empty( $args['birth_date'] ) ) {
		// Birth date as a Unix timestamp.
		$then = strtotime( $args['birth_date'] );

		// A person 18'th birthday as a Unix timestamp.
		$adulthood = strtotime( '+18 years', $then );

		// Current time.
		$now = time();

		if ( $now < $adulthood ) {
			UM()->form()->add_error( 'birth_date', __( 'You should be over 17 years old.', 'ultimate-member' ) );
		}
	}
}
add_action( 'um_submit_form_errors_hook_', 'um_custom_validate_birth_date', 30, 1 );

This gist is a part of the article Restrict registration for under 18 years old

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/