Guess all the other mules died from old age!
Or corporate downsizing has gone too far!
Really, I don't understand the question. Why ask what is the oldest then exclude old brands? (By the way 20 Mule Team Borax is not a laundry detergent).
But:
1. If we're talking about the oldest company that makes a laundry detergent, or the detergent that has been on the market the longest, the answer would be, for the USA, Procter & Gamble and their Dreft line, which was the first synthetic laundry detergent, introduced in 1933.
2. Other laundry products for washing clothes have been on the market longer, but were, like Persil, soap originally, not detergent. Ivory Snow and Fab both go back before 1910.