"In the early betas of iOS 9, Apple had activated this feature for older iPhones. Users would just place both their thumbs on the keyboard and it will act as a trackpad. However, Apple removed this feature for unknown reasons. Right now, no other iPhone other than the 6s and 6s Plus can perform this feature which is really annoying."

http://wccftech.com/turn-keyboard-trackpad-3d-touch-iphone-6...