Ask your airline, not the airport. Both Istanbul Airport and Turkish Airlines say to request wheelchair or special help when you book, or at least 48 hours before the flight. Turkish Airlines says later requests are met only if staff are available. Ask for help on every flight of your trip, including the connection, and ask the airline to confirm it in your booking. For help from the airport itself, call its 24 hour call center on 444 1 442 (a Turkish short number that may not work from a foreign phone), or email ist@igairport.aero. The airport also lists a WhatsApp line, but its pages show two different numbers (+90 444 1 442 and +90 549 563 34 34), so check which one works.
Airport staff in yellow vests are there to help guests aged 65 and over. Assistance call phones are on the guide paths from the metro station to the terminal entrance gates. Special Assistance Service Points are in the departures check-in area, across from the first and sixth terminal entrance gates. The free buggy stops and call points are described under 'Getting around'. After landing, go to the transfer desks opposite the international transfer security checkpoint.
Free for guests 65 and over: the buggy, and the Fast Track security service at the points the airport lists. Ask staff whether Fast Track is open at your transfer security check. Turkish Airlines says its wheelchair service is free for approved requests, but we could only see this in a search summary of its page. A paid Meet and Greet helper is available (35 percent off for guests over 65): book through the call center or igapass.com. A Sunflower Lanyard lets staff see you may need extra help. If you use a wheelchair or have a pacemaker, tell the security staff before you are screened. The airport says they use other search methods for you.