This is such a murky issue, made all the more muddled by the conflation of image creation date and card manufacturing date into the concept of "oldest." You can sometimes specifically attribute an image to a date or year based on various factors that prove when it was taken but that doesn't prove when the card itself was made. Unfortunately, there were many images that were pirated in the 19th century, especially as the first wave of photographers died out, went out of business, or retired. A lot of late-19th century CDVs and cabinets reuse earlier images shot by famed photographers who were by then unable or unwilling to protect their creations.
The copyright system helps a bit, provided there is copyright data on the card. The earliest copyrighted images were sent to the US District Courts serving the cities where the photographers were located and had the dates of image submissions placed on their fronts along with a tedious copyright notice that listed the district court and the photographer's name. That system ended in July 1870 when the whole shebang was centralized in Washington DC. So, if you find a card with the USDC-style copyright notice on its front, you can pretty much conclude that the card itself was made pre-July 1870, who made it (as listed), and that the date listed on it was when the image was shot. Absent that, you have to do leg work to find out when the image was made, which at least tells you it can't be any earlier than that date, and you can sometimes cap the image creation date based on the death of the subject (for example, I know John C. Heenan, the subject of the first boxing card, croaked in 1873, so it was highly unlikely that he'd have sat for a portrait after that time). But pinning down the specific date of manufacture is often impossible. It is a lot of supposition and research and there is always a possibility of error.
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