As the title suggests. :) What's the earliest model of the iPhone to support this specification?
The earliest iPhone models to support 802.11ac were the iPhone 6 and iPhone 6 Plus, both introduced in September 2014. As for iPads, the iPad Air 2 was the first iPad model to support 802.11ac - introduced in October 2014.
You may find the following links useful:
- Apple - iPhone 6 - Technical Specifications*
- Mactracker (comparing the specs of iPhone models shows that the iPhone 6 range was the first to include support for the 802.11ac specification)
- Wikipedia (see first quote below)
- Every Mac (see second quote below)
* This link takes you to the Web Archive of Apple's iPhone 6 specifications page as it was in September 2014.
According to the Wikipedia entry for the iPhone 6:
The expanded LTE connectivity on the iPhone 6 and iPhone 6 Plus is improved to LTE Advanced, with support for over 20 LTE bands (seven more than the iPhone 5s), for up to 150 Mbit/s download speed, and VoLTE support. Wi-Fi performance has been improved with support for 802.11ac specifications, providing speeds up to 433.0581 Mbit/s—which is up to three times faster than 802.11n, along with Wi-Fi Calling support where available. The iPhone 6 and iPhone 6 Plus add support for near-field communications (NFC). It is initially used exclusively for Apple Pay—a new mobile payments system which allows users to store their credit cards in Passbook for use with online payments and retail purchases over NFC. iOS 11 added limited use of near-field communications besides Apple Pay for 3rd party apps.
Source: iPhone 6, Wikipedia. [Bold emphasis mine]
Finally, according to EveryMac.com:
All of these iPhone models support GSM/EDGE (850, 900, 1800, 1900 MHz) and at least 802.11a/b/g/n Wi-Fi (the iPhone 6 and iPhone 6 Plus also support 802.11ac with speeds up to 433 Mbps), as well as Bluetooth 4.0. When releasing iOS 9, Apple upgraded the Bluetooth capability in the iPhone 6 and iPhone 6 Plus to Bluetooth 4.2, but it only supports improved privacy rather than increased speed.
Source: Differences Between iPhone 5/5c/5s and iPhone 6/6 Plus, EveryMac.com [Bold emphasis mine]