In this era old hardware has become a misnomer. Old hardware is not necessary slow. 6 years old is 2009 and DirectX11 was already existing. We still don't have much newer today.
You need to consider two things:
This is the level of instructions that the hardware will support, DirectX9,10,11, or OpenGL something (this depends on the drivers). And the CPU instruction set: SSE4, AVX..
This is the biggest headache usually in developpment, scalability facing those is harder than scalability facing power. DirectX9 had something called
technique in the
.fx files to fall back, for both points. But for CPU instructions you need multiple executables, or large branching sections in your program. Or don't use them.
Luckily today, even 6 years is a short period and you will have access to about everything you need even on "old" hardware.
This is the point that should scale, on todays machine, and not legacies machine. A high end of 2009 potentially features a geForce GTX280, this is very powerful, and much more powerful than your low end AMD A6-3650 APU or something.
One need to understand the gigantic gap of performance between low end, and high end, of today's machine. Because it largely exceeds the gap of performance of your 6 year range machines on the high end scale.
So its not a consideration of market regarding renewal, its a consideration of market regarding original buying budget. And if you need to target today's laptops or low end Desktops, you are going to need to scale anyway, and way more than you think.
Try to care about resolution, pixel fillrate and graphics memory bandwidth. You play on those factors thanks to the number of screen passes, the width of your Gbuffer if you have deferred shading (very costly on low end machines), the resolution of your textures and their quantity present on screen at any given time t...
Polygon richness will not be an issue unless you are targeting 2002 hardware. Shader ALU/FPU complexity will not be a problem either, unless you target 2005/2006 hardware.