I don't often do a bunch of pieces art. Even with older kids. I tend to cut out a shape ( or they cut if they can) that matches the theme and have them do a collage, add stickers, watercolor paint, wheel paint, etc.
Since it's almost always process art my kiddos start as soon as they seem interested, which tends to be about 16 mo. Sometimes sooner, sometimes later.
If there are pieces all the children do it anyway they want. Here's some examples. The triangle Santa does have pieces but possibly all the pieces will be stuck in one pile if a young child is doing it. I only do something like this a few times a year.
The bunny Would be made by golf ball painting. These are pictures I've saved from the internet but are things I've done. I wouldn't use glitter but I'd use glitter paint from discount school supply. I love glitter paint! I use a lot of bright or neon construction paper and/or paint.
The jack o lantern has pieces but it's very simple! I always put glue in the paint so pieces stick wherever they place them after it dries.
The Pom pon prints on egg shape would not be this neat! Oh, darn! Only one thumbnail showed up! Grrrr!