Will it be helpful if I solve old questions of IIT entrance exam to get good marks in IIT JEE advanced exam 2018?

Yes, I think that it would very useful if you solve old year's IIT papers. But, I also think that you should try some other questions which are related with your topics. You should also solve other entrance exams papers. It will be very useful to know which types of questions are asked in the entran

It should be helpful for you. But, I want to tell you that in 2016th of JEE advance exam, most of the students are blaming that the whole paper was out of course. You should try and practice some new questions.