Point Density - particle age colour

I have been playing with the point density texture today - and cannot get it to work right.

I want my particles to change colour with age - however when I set the “colour source” to particle age and run it through a colour ramp - rather than the particles changing colour with age (like in the video below - I have used an identical node setup) - they seem to have a coloured halo that corresponds to the colour ramp instead and keep that colour right throughout their lifetime. This screenshot is taken at frame 54 of my animation - the particles have a lifetime of 50 frames from the point of emission.

This is the tutorial I followed and the result I expected:

"they seem to have a coloured halo " - as it was said in video - these are voxels.
Looks like correct result for the Age to me… http://www.pasteall.org/pic/show.php?id=105166

Edit: Actually, if particle age is set to 100 and they cover all color range in 50 frames…Not quite right?