Hello Aspirant,

Yes,you can join IIT/NIT by re-appearing JEE again in the next year and meanwhile you can pursue B.Tech at some other college but you have to start fresh again(from first year) if you change your college,also provided you have not exhausted your chance of appearing JEE.A candidate can appear JEE Mains for 3 attempts in three consecutive years and can appear JEE advanced twice for two consecutive years.Although there is no age limit in JEE Mains JEE advanced certainly has a age limit as per 2020 eligibility criteria a student born on or after October 1, 1995 is eligible,5 years of relaxation is given to SC/ST/PwD,in 2021 this criteria will increase by one more year.

Hope this was helpful

Regards