I flew from Geneva to Istanbul with Turkish last month and was only able to select my seat at online check in, 24 hours before departure. I think it might depend on what ticket you have; with some tickets you can choose your seat at the time of booking, with others you have to wait for check in. That's the case with Swiss too.
That was the first time I'd flown with Turkish. The flight out was on a brand new 737, a lovely plane with lots of leg room, excellent service and seat-back entertainment (well, I like to look at the map).