That's kernel version and the kernel version is generally tied to the router main board used in each model and the development SDK that is provided with it. If the board manufacturer hasn't updated the SDK by now they aren't going to. They will be devoting current effort on the newer technology hardware they are selling now.
If Cake needs features available in newer kernels then that's not going to change since the kernel won't change.
Whether it's possible to implement the bits needed using older kernel interfaces is the question and the answer is probably no since the functionality available in the kernel is also often closely tied to the hardware on the router main board.
