We've started when DS was old enough to stand up at the kitchen bench, so from around 18 months. He's 2.5 and loves to mash potatoes, put chopped veggies in a roasting pan or salad bowl, he tries to chop them too with his 'little knife'. He also spreads things on his own toast. I think they do quite a bit of food prep at CC as well, as well as serve themselves lunch. DP drops him off and told me that he's been pouring his own cup of milk for morning tea for about a year. I didn't even know he could do that, LOL.
So yeah, i think the earlier the better and it just naturally progresses.