What is the reason for the absence of a "prayer callus" (zebibat al-salat) on the heads of many sheikhs, given its appearance on the questioner's head at the age of sixteen, and what is the interpretation of Allah's saying: "Their mark is on their faces from the trace of prostration"?

The appearance of a prostration mark on the forehead is not always due to frequent prostration; it could be due to variations in skin strength and weakness. The preponderant opinion is that "their mark is on their faces" (referring to the verse "سِيمَاهُمْ فِي وُجُوهِهِمْ") signifies the light of obedience, worship, humility, and good character, and not necessarily rough skin. The visible mark on some people's faces might be due to their thin and sensitive skin, and not necessarily a sign of righteousness, as this mark may not appear on the face of someone who prays frequently.