Do airlines charge for wheelchairs?
No, they do not. This is because they follow the FAA regulations very closely, and are not allowed to do this. Airline flight attendants will also usually provide you with someone who will help wheel the elderly person(s) into his/her seat.
Where will they store the wheelchair?
Most airlines have space reserved for folding wheelchairs, and they will store it in that area. If you have a wheelchair that may require some dis-assembling then the airline is responsible for returning it to you in its original condition.