Jeremiah was very young when he was first called by God. I don't think it is certain but some say he was in his early teens between 14-19
hes 21
Jeremih is 21. I'm not sure when his birthday is but I read that he is 21.
He turned 17 in March.
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Technically, according to the Bible... Jeremiah was a prophet before he was born:"...Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and...