For Para SF there is nothing like filling form and all, you will be selected based on merit. If you want to become a Para SF cadet you must first be a Para trooper which is a 3 month training. After that you will be selected on merit. It can be done for JCOs and even from officers. Then you have to go for a 6 month hard training. After that you will be sent on 1 year probation in a battalion of para forces. You will be selected based on your physical and medical capabilities. There are many criteria but first is to join Indian Army. Age doesn't matter but yes you should be young, what matters most is your skills are.