It is the prime responsibility of children to obey the parents and seek their pleasure. The children should avoid actions that may cause the anger or displeasure of the parents. Therefore, the boy and girl should marry according to the wishes of their parents after one year when their parents are convinced.
However, if grownup boy and girl marry their match in front of two witnesses then Nikah will be valid. The boy and girl or their representatives must be present in the meeting of Nikah. Therefore, in case of both being at distance, it is better that the girl appoint someone as her representative to get her married, then the representative proposes for nikah on behalf of the girl before two witnesses in the meeting of Nikah and the boy agrees to it.
One more thing, you should notice that performing marriage secretly is disliked in Islam. The Prophet (Sallallahu Alaihi Wasallah) said: اعلنوا ھذا النکاح واجعلوہ فی المساجد (Publicize nikah and perform it in mosques). Because, when the nikah will take place in secret, the people will have suspicion about the relations of the boy and girl. And, if there is pregnancy, they may feel difficulty in proving it legal. The parents also will be ashamed and this will invite their anger.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband