According to jee mains 2018

Only those candidates who have passed their Class 12th Exam or any equivalent qualifying examination in 2016 or 2017; or those who are appearing in their Class 12th Exam or any equivalent qualifying examination in 2018 are eligible to appear in JEE(Main)- 2018. Candidates who passed Class 12th/Qualifying examination in 2015 or before as well as those who will appear in such examination in 2019 or later are not eligible to appear in JEE (Main) – 2018. Candidates who appeared in class 12th/equivalent qualifying Examinations in 2015, did not pass in 2015, but passed in 2016 are also not eligible to appear in JEE (Main) 2018.

Yes u can give in 2019 as apperaing with another board