The particle’s age divided by its finalLifespanPP. The finalLifespanPP attribute is used for the particle’s age. This works in all Lifespan Modes of the particle shape. But if Lifespan Mode is set to Live forever, all lifespans are effectively infinite, so normalized age is always zero.
Out UV Type set to Normalized age is the default; you don’t need to set a use relative age attribute as you did in previous releases.
You can use this output of the Particle Sampler Info node regardless of whether the particle shape has an ageNormalized attribute.