Jeremiah is the longest of all the sixteen OT prophetic books. His age at the time of his call is likely somewhere around 16 years old. Imagine that; being called by God to serve as a prophet in Israel as a teenager. And if that wasn't enough of a bombshell, his calling was not just to speak for God to Israel, but "to the nations." He would serve as a prophet for more that 40 years, throughout the reign of three kinds in Judah, at a time of great reform, then a time of instability, right up to the time that Judah would be taken into captivity in Babylon. His calling was somber; it was grim. These were not the best days for his people. When Jeremiah is called it is a time of chaos in Israel; a lot like ours.