sow your wild oats go through a period of wild or promiscuous behaviour while young.

☞ Wild oats are weeds found in cornfields which resemble cultivated oats: spending time sowing them would be a foolish or useless activity. The expression has been current since the late 16th century; from the mid 16th to the early 17th century, wild oat was also used as a term for a dissolute young man.