Hello Callandor, and welcome to the Martial Talk Forum!
As Master Stoker (Terry) asked above, it will be helpful to know where you are.
I already paid a life time membership fee to the association in our country which is affiliated to WTF. Is this enough to participate in the testing?
It will also be helpful to know what organization it is that you joined for life-time membership. However, chances are that this fee is for record keeping in the event that you do test, and does not automatically grant permission to test.
Is it allowed in WTF TKD to take promotion test even if not enrolled at a school?
First point of clarification I want to make is that the WTF (World Taekwondo Federation) has absolutely nothing to do with promotions. This arm of the "Kukkiwon" (World Headquarters for Taekwondo in Seoul, Korea) used to keep record of Black Belt "Dan" (degree) promotions only, but they no longer do that. The WTF is solely responsible for Taekwondo competitions (and various aspects thereof).
Instructors, and/or their respective National organizations to which they are affiliated keep track of color belt promotions and issue color belt and Black Belt certificates (Black Belt certificates may also be obtained from the Kukkiwon upon recommendation by a certified Instructor who has this authority).
in our place, exams are not held in a school but in a neutral venue where different TKD schools participate. Could I just show up during those schedules even if not enrolled in a particular school?
Not likely. Testings are procedures in which instructors (usually Master Instructors and/or Grandmasters) observe each student's performance on a particular day, discuss with their daily instructor the student's capabilities, attitude, and progress, then verify that the student is ready to advance to the next level of learning. Only by being enrolled in a school with an instructor guiding you, is participation in one of their tests usually permitted.
There are occasions where a student can receive remote instruction via regular communication of videos, phone conversations, e-mails, and then personal visits on a less frequent basis. Sometimes, testing with a legitimate organization can be done through taped videos of your performance being sent to a Master Instructor who has the authority to approve your promotion through the color belts.
One option you might have in your current situation, is to go your instructor and explain what your personal needs and desires are (given your age and any physical health concerns). Most good instructors will teach you everything you need to advance without forcing unnecessarily difficult training on you. This does not mean that you would get to pick and choose everything you do, but your instructor can adjust your training to meet your needs, yet not delete important aspects. You don't have to be able to kick above the waist, but if you can try, kick as high as your body will allow. Your instructor should be able to allow you to move to an area of the dojang and practice forms, or some other valuable aspect while the other students spar or do the extreme training.
Most of your training can be kicking low, and doing forms, one-steps, hoshinsul (self defense tactics), hapkido, and whatever physical fitness and health exercises you need to stay in shape. If your current Instructor won't accommodate you, check with one of the others in your town. If they won't help you, get back with one of us here. Perhaps we can get you on the right track. I don't recommend training only at home without some kind of connection to a good, legitimate teacher.
Please reply with the answers about where you are, and with what organization your school is affiliated.
CM D.J. Eisenhart