You could respectfully ask them if they would sign for you to marry someone you love right now? If not, why not?
Help them to reason that marriage is less permanent than Baptism and considered by the society as the MOST important decision in a persons' life. Let them know you don't feel comfortable just yet.
After all, the Bible never says how old a person should be to marry but it says "past the bloom of youth" Likewise, the Bible doesn't say how old to be baptized either except that "The Greatest Man who Ever Lived" waited until he was 30. Only MEN have urged young baptism not the Scriptures.
I hope this helps.