What do you mean by proposal of nikah? If both the boy and girl are mature and present in the meeting of nikah, and they make ijab-o-qubool (proper exchange of words) of nikah in the presence of two witnesses, the nikah shall be valid and effective. But, if the girl is not present in the meeting of nikah, her wakil or wali should be present in the meeting. When the girl becomes mature though no her guardians (father and grandfather) cannot impose their choice, but it is inappropriate and against the modesty for a girl to marry herself without the permission of her guardians. However she can live with that boy after a valid nikah takes place.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband