B.Com

First I doubt if you are eligible to study a pure computer Science after B.Com. Your education background has to be relevant when you are applying abroad. You should match the desired course basis your interest and education background.

Yes, students appearing for final year of Bachelor's degree/equivalent qualification examination and graduates can apply for CAT.

Yes, engineer's calculator is allowed in the examination.

Best will depend on what you want. There is no such 'best course', it depends on what the student wants and if the student has chosen the right course for him/her according to her interest and career plans. You have many options after B.Com like Master of Commerce, MS Accountancy, MS Finance, PG Dip

For those destined to become an MBA holder, any graduation will do. Any graduate with 50% aggregate through the clearance of eligibility test such as CAT, MAT, ATMA-AIMS, CMAT, etc. As applicable to the institute of study become eligible for admissions to an MBA programme. For those with a B.Com deg

Normally, a prospective applicant must be a graduate having secured at least 50% marks in aggregate, from a recognised university and have cleared eligibility test such as CAT MAT CMAT ATMA AIMS or equivalent as applicable to the institute of study. As per CAT 2018 eligibility criteria, a candidate

After having a look at your percentage, it looks pretty tough for you to get admission in MBA. Whereas MBA requires at least 50%- OBC/General and 45% for SC/ST candidate to qualify. Saying about perfect age to study, there is no age bar for study you can learn anything at any age.
Best of Luck for y