could be wrong but from memory 18mph is minimum for any real advantage.
You may be, I think it’s 12mph. Most riders in the Alpe D’Huez TT a while back used aero bars as above 12mph the drag reduction offset the weight (Cervelo did a study/report on it).
A lot of XC involves too much movement and variation in position for me to have interest in aero gains but I guess it counts for something on gravel bike type of terrain at 15mph or so. For longer sections / race, maybe worth it. I remember being sure to be in a low tuck for every fire road descent on a long low-tech race once to save energy and gain a bit of speed, also wearing lycra shorts and close-fitting sleeves in part for the cumulative benefit over the usual loose-fit top and shorts.