If the 14 year old girl is mature as per the Shariah i.e. she began her menses, then she will have right to conduct her matters herself, she will not need permission from her guardian, she can marry herself. But a Nikah performed with the consent of parents has more blessings. The marital life becomes prosperous and happy with the duas and compliments of parents.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband