Ref. No. 1284
In the name of Allah the most Gracious the most Merciful
The answer to your question is as it follows:
If the boy and girl are mature and do Nikah as per the shariah ruling with mutual agreement, in principal the Nikah will be valid and applicable. However, they ought not to go against their parents.
And Allah knows best
Darul Uloom Waqf Deoband