To prevent aging of the hands, I always use sunscreen and reapply after washing hands (even if the sunscreen is waterproof, I do not believe any sunscreen is fully waterproof). To do this,I buy hand lotions that have UVA/UVB protection and antioxidants, kills two birds with one stone, you get sun protection plus additional anti-aging ingredients
To help prevent sagging of the neck, I would, of course, always apply sunscreen and use products that contain ingredients known to maintain skin collagen like Vitamin C and/or Retin-A.