Hello student I'm sorry to inform you but no you can't join ISRO in class 5 however you can can apply for "Young Scientist Programme" "YUva VIgyani KAryakram" after passing class 8. The programme will be of two weeks during summer holidays it's proposed to select 3 students from each state /union territories.

For more details refer to the link mentioned below - https://www.isro.gov.in/update/26-mar-2019/young-scientist-programme-yuvika-online-registration

Feel free to comment if you've any doubt

Good luck