Water problems dont come without dropping leaves, either the plants get too much water and leaves are too heavy for the plant or they become too weak to hold their own structure, this happens inbetween hours of wrong waterings.
You can check where on the plant it occurs, if its next to your light, its the light. But then all leaves should look the same, if there are healthy ones next to it, its something else.
Normal age signs would appear on the oldest leaves, the ones that are the lowest on your plant, not on the canopy
Its probably a deficiency, wouldnt say its potass UNBF UANFA what ever the word is, its phosphorus i guess.