What are the best OTC products to use to combat skin aging?

Sunscreen. By far, one of the main contributors of skin aging is uv rays (the sun, tanning beds). So sunscreen should be a part of your otc regimen. Aside from that, retinoids, which are derived from vitamin a, boost collagen production and increase skin cell turnover. This improves fine lines and wrinkles, discolorations, and acne. It's available by prescription, but also over-the-counter (otc) retinol.
Proper diet. The skin is the largest organ in the body. So----it requires good nutrition like all the other organs. Its cells are produced from the primitive embryonic cells that are used to make the brain, so---take care of it as above.