The Chronicle of Fredegar says he led an invasion of the kingdom of Burgundy in 604, where he was defeated and taken prisoner. If so, he must have been only a child. His father was born in 584, so Mérovech cannot have been born much before 598.

-=-=-=-=-=-=-=-

In 606 Theuderic is said to have had a son, Merovech, to chom Chlothar II had stood as godfather. This ceremony made Chlothar Theuderic's 'co-father'. Chlothar himself had been saved from loss of his royal inheritance and probable death as an infant when his royal uncle Guntram became his godtather. Yet, unlike Guntram, Chlothar did not secure Merovech's succession after Theuderic's death in 612.
https://books.google.com/books?id=g6yq2sKLlFkC&pg=PA363&lpg=PA363&d...