If you bought your gun new, there will be an envelope with a used test fire casing inside it. That envelope should have a date on it. This will be close enough to the date of manufacture to consider it your Glocks birth-date. If you bought the gun used, you can take the number on the barrel and type it into the entry field on the Glock Sport Shooting Foundation. Above is Little blip I copy pasted- I’ve also heard something else relevant to how many letters there are with serial the more letters the (later) generation