By the time you get to the late 16th/early 17th century, stays are like bras - people will look at you funny if you don't have them. That isn't to say you need a fully boned exact replica of the effigy stays but you do need something for support and to give the correct silhouette.

Kids were often still in kirtles because no one would risk their sanity trying to make a new pair of stays every three months for their growing child.

Layers: Shift, farthingale/bumroll, petticoats, pair of bodies (Stays), skirt, jacket, apron.