Hi, thank you for posting on HCM, unexplained pain in the left arm is significant as it may be a early sign of ischaemic heart disease due to referred pain from heart. However if you are young and otherwise healthy with no co morbidities like diabetes , high blood pressure, etc cardiac event is bit unlikely. Most common cause for this type of dull aches in arm is muscular pain at any age and which is not a major problem. Which usually settled with analgesics. But because you have left arm pain it s prudent to visit a doctor urgently and exclude any heart problems. Hope the answer is helpful. If you have any more concerns pls feel free to ask. Thank you.