Question:Is there a scripture in the Bible that says you should marry a partner of your youth?
No, but there are passages which state acknowledge that most people marry when they are relatively young.
"Let your fountain be blessed, and rejoice with the wife of your youth" (Proverbs 5:18).
"Yet you say, "For what reason?" Because the LORD has been witness between you and the wife of your youth, with whom you have dealt treacherously; yet she is your companion and your wife by covenant. But did He not make them one, having a remnant of the Spirit? And why one? He seeks godly offspring. Therefore take heed to your spirit, and let none deal treacherously with the wife of his youth" (Malachi 2:14-15).