Kernel version requirements:

• All SoCs productized in 2017 must launch with kernel 4.4 or newer.
• All other SoCs launching new Android devices running Android 8.0 must use kernel 3.18 or newer.
• Regardless of launch date, all SoCs with device launches on Android 8.0 remain subject to kernel changes required to enable Treble.
• Older Android devices released prior to Android 8.0 but that will be upgraded to Android 8.0 can continue to use their original base kernel version if desired.

Source: AOSP