Hello Dredgyh.

The lower age limit for JEE Main is 17 years and upper age limit is 25 years.So,yes ,if you give mains next year and score a good marks ,you can  definitely get into NIT.But ,if you will be giving mains next year ,you will be completing your 2nd year hopefully at BIT mesra.If you were to take my opinion,I don't think it would be  a wise choice to spent 2 years of engineering in waste ,just to get into NIT.BIT mesra is a very good and renowned college.If you are giving mains this year ,i.e. 2019,then you can think of leaving BIT and going for NIT but for  next year,I don't think it would be wise.

Hope this helped you in your doubt.Good Luck!