Pacemakers, bone replacements, etc. shouldn't affect the SSSS likelihood as that is determined before the security line.

Saw two elderly pax with SSSS on UA44 today, the LAX-IAD redeye.

This is no doubt a simplification, but elderly pax are almost by definition not flying for business and are probably less likely to have FF accounts. This makes them unique/discrete flyers in the computer records, raising a flag and making them more likely for SSSS..