When was trim on airplanes used for the first time?
Although Anton Flettner patented the trim tab in 1918, (as said in a comment above) it seems that 1910 was the first implementation of "trimming" (according to the following article, linked below)

https://www.flightglobal.com/pdfarchive/view/1910/1910%20-%200255.html

Although it was not adjustable in flight, it still seems like the first time any concept of trim was introduced was in early 1910. However, Anton Flettner produced the first ADJUSTABLE trim tab.