I started DD1 on packing away toys at about 3.5 years old. I found before that, it was more of a struggle to get her to help than was worth it. I did start out with little bits and generally increased what I expected her to help with as she got older, as in, I try to keep it age appropriate. DD2 is only just about to turn 2 so she doesn't help with pack up generally, but, because we are all cleaning up she might help (might just as likely make more mess too…) Still, she does follow the example generally and when she gets a bit older I will certainly give her a fair share of pack up responsibility.

At 4 and 5 I would absolutely expect some pack up assistance.

I find what works best is to break down the task - Can you pack up the puzzles while I do the books? How about we start with the Dolls clothes together? Can you tidy up your table? etc
I also have a clear designated place for everything so the kids know exactly where things belong.
The other thing I use is a pack up song. I bought a song on itunes, "pack up time" by the Mudcakes. It is brilliant, a fun upbeat song to race around the room cleaning up to. It works a treat with everyone singing and cleaning up together and when the song is done, we generally have the house back in order. The Mudcakes are a local Melbourne kids band too so I am doubly happy supporting Australian music