Everything known about Musaylima comes from the testimony of the people who defeated him, Islamic accounts written after his death at the battle of Aqraba in 632 or 633, concerned with his revelation, his following among the Banu Hanifa and his death rather than his early life. No source records when he was born.
What would resolve this A source independent of the Islamic accounts that defeated and denounced him, from within the Banu Hanifa's own oral or documentary memory, naming a birth year or generation for Musaylima.
OpenEarly Islamic history, pre-Islamic Arabian historyP. Bearman, Th. Bianquis, C. E. Bosworth, E. van Donzel and W. P. Heinrichs (eds.), The Encyclopaedia of Islam, 2nd edition