There is no age limit to appear MHT CET Entrance Exam. As per the eligibility you should have minimum 50% aggregate marks in 12th standard examination (45% for SC/ST/OBC candidates). You should have completed your 12th standard or equivalent examination from a recognized board/university with Physics, Chemistry and Mathematics. If you meet the eligibility then you can apply  You can check the further eligibility to apply for the exam with the given below link.

https://engineering.careers360.com/articles/mht-cet-eligibility-criteria

To check the documents required at the time of admission, You can go through with the given below link.

https://engineering.careers360.com/articles/mht-cet-counselling

Good Luck!

Yes,

You can take admission in Btech if you get enough marks in your entrance exams. Once you are alloted a college, you can get your admission cancelled in your existing college and collect your documents if any, submitted. Then you can take part in the counseling process of your alloted college.

Goodluck.