The text of Gen 5 says this:
21 When Enoch was 65 years old, he became the father of Methuselah. 22 And after he had become the father of Methuselah,
Enoch walked with God 300 years and had other sons and daughters. 23So
Enoch lived a total of 365 years. 24 Enoch walked with God, and
then he was no more, because God had taken him away.
To this record must be added two more texts about Enoch:
- Heb 11:5 - By faith Enoch was translated not to see death, and he was not found, because God had taken him up. For before the translation, he was commended to have pleased God. [The text of LXX in Gen 5:22 has "well-pleasing to God ..."]
- Jude 14 - And Enoch, the seventh from Adam, also prophesied as to these, saying: "Behold, the Lord has come amidst myriads of His holy ones
Thus, it appears that Enoch was called to a be a prophet of God after the birth of his first son. Just why God chose this timing is not stated. However, here are some suggestions:
- This was God's sovereign will
- Enoch, following the birth of his son, Methuselah, re-dedicated himself to serving God
- God waited until after the birth of Methuselah so that Enoch would better understand the coming and birth of Messiah
Thus, there is nothing here to suggest that before the birth of Methuselah, Enoch was wicked or needed drastic reform. All that is stated is a greater dedication and closeness to God at that point in his life.