Question:

Is there a scripture in the Bible that says you should marry a partner of your youth?

Answer:

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).