One plausible physical explanation for IF is it promotes autophagy. 18-22 range probably doesn't gain much if any AIUI since you system is already running near

One plausible physical explanation for IF is it promotes autophagy. 18-22 range probably doesn't gain much if any AIUI since you system is already running near optimum if you just eat regular times and don't snack around the clock. But older? Maybe the human body takes longer, in general, to switch on autophagy as it ages, and the additional time in the food-gap helps.