Add a boost function, to prioritize recent content in Search API Solr queries. More recent articles will get boosted heavily, older ones less and less.

boost-recent-content-with-searchapi-solr.php

<?php

/**
 * Boost published_at field to prioritize newer content.
 *
 * @see https://wiki.apache.org/solr/SolrRelevancyFAQ#How_can_I_boost_the_score_of_newer_documents
 * Implements hook_search_api_solr_query_alter().
 */
function my_module_search_api_solr_query_alter(array &$call_args, SearchApiQueryInterface $query) {
  // Boost more recently created articles. Boost the field with score +8 shrinking during 365 days.
  $boost_field = 'ds_published_at';
  $call_args['params']['bf'][] = 'recip(ms(NOW/HOUR,' . $boost_field . '),3.16e-11,0.5,0.5)^800.0';
}