Very oldest is boring and, while I know the “when”, not usefully specific as to era: a United States Marines ad, with the “halls of Montezuma” song being sung by men in uniform. Circa 1962-3, when I was 3-4 years old, on some Tallahassee FL TV station.
I remember lots of cigarette ads: “to the sailor, it’s the ___ ___ ___, to the ___ ___, it’s the ___ ___ ___, to the smoker, it’s the Kent.”; the “Winston tastes good like a !!!, !!! cigarette should” ad; the Camels ad with the guy showing the holes in his shoe soles after walking a mile for a Camel;
Remember a lot of doll ads for girls: “Baby crawl-a-long, baby crawl-a-long, look she crawls right across the floor” / “Little Kiddles” / “Malibu Barbie and her suntanned friends” / “Lucky Locket Kiddles” / “Flatsies, Flatsies, they are flat and that is that”…
Gizmos and other stuff: “Kenner’s See The Show Projector” / “Creepy Crawlers, Creepy Crawlers” <in a whisper> / “Meet the Swinger, Polaroid Swinger…take the shot, yeah yeah, blah blah blah, yeah yeah, then take off!” / “Be a Betty Crocker baker bake your very own cake in your Betty Crocker Eazy-Bake oven!”
Food ads: “N. E. S. T. L. E. S., Nestle’s makes the very best…chooooh klaaat” / “Oh I wish I were an Oscar Meyer weiner, that is what I’d really like to be eee eee, cuz if I were an Oscar Meyer weiner, everyone would be in love with me” / “Taste that beats the others cold, Pepsi pours it on” / “Tall up! For the tall tall taste of Kelloggs golden flakes! It’s corn that you can really crunch, corn that really packs the punch, tall American tall American Kellogg’s corn flakes!”
Hmm, now I more clearly remember why I quit watching TV… stupid commercial earworms…