Hi Vidyuth,
There is no set cut off, but as you get into the early to mid 30's there are fewer and fewer MBAs in full time programs at that age, it is true. You can view class profile data on any school's website to give you a feel for it.
Re: GMAT, I would suggest setting aside 6-8 weeks (only!) for prep. A private tutor is most effective as they can literally watch you approach problems and give you very specific advice. A classroom teacher would be 2nd choice, online class 3rd choice. Be sure your prep includes several full length, realistic, computer adaptive practice exams along the way, and then study from those to see how you can improve. Do most of your prep on the computer. Work on timed practice sets. Analyze your performances to identify where you need the most work. Make sure you have a strategy for every question type. Work on how you'll approach the scratch work. etc. Good luck on everything!