I have six month older Google adword account but I formed some day before MCC account. May I appear in adword professional exam?
I don't really get what you're saying here - are you asking if you can take the adwords professional exam? If you are then the requirements are here: https://adwords.google.com/select/ProfessionalWelcome It says "The 90-day period for accruing client account spend commences at the time the account is linked to the MCC.", so yes, you have to have MCC (and meet all the other requirements).
You can take the exam anytime. But in order to fully qualify for the Google Advertising Professional Logo, you need to complete other requirements too.
I have registered for the Google adwords exam in march. In my MCC the requirements ststus is MET, so does that mean I can give the exam now or have to wait for 90 days to finish?
If you enrolled to the GAP program with the current MCC account (and spent $1000) all you have to do is to pass the exam.