I used to live there and i always thought it was 18 - Turkish people might bu muslim but you will find that they will drink just as much as the non muslims. My Turkish landlord used to drink alot of raki and even taught me how to drink it properly.
I've seen tourists as young as 12/13 being sold alcohol in Turkey and also very young people working behind bars. There's a very relaxed attitude but Turks are Muslem and really don't approve of drunkeness (especially drunk women). There's also the risks of getting drunk when you're abroad to take into account. Enjoy your holiday