Hello Jay,
As you are a diploma student and its your last year in diploma and you want to pursue your career in MBBS medical.
Yes,you can give 12th Science exams after your diploma but first you have to take admission in 12th in science stream and after that for getting into MBBS you also have to clear NEET exam.
If you pass the 12th board exam from science stream and also qualify NEET exam for getting into MBBS by achieving whatever the cutoff is there for MBBS  then you will get MBBS seat.
Any person of any age who is willing to give 12th exam in any stream can give it.
Hope this may help.you.