I'm trying to do a smoke trail effect where after around 4-5 seconds the end will slowly disappear instead of just suddenly being gone and so far I am unable to achieve the effect, I'm thinking it has something to do with @age and a color node?

Might need to know a little more about how you're creating the effect - is the POP network feeding a smoke simulation?

But in general, if you want particles to fade at a certain point in their lifespan, a POP Color node will do it, though you'll probably want to set the Alpha (opacity) rather than the color. There's a little dropdown box next to the Expression box that'll put in a “fade based on age” expression for you so you can see how it works.

Don't forget to set an appropriate lifespan for your particles in the Particle source node (wherever you're birthing the particles).

Thank you