Separating gets confused with two other complaints, and the fixes are different, so it is worth being precise about what you are seeing.
Separating is the foundation film breaking apart into patches, with bare or shiny skin visible between them. It usually starts on the nose and chin and shows up in the afternoon. Creasing is foundation gathering into a line — a smile line, a crow's foot — while the rest of the film stays intact; that is why foundation settles into wrinkles. Pilling is product rolling up into tiny balls during application rather than hours later; that is why primer pills.
The four causes of separating, in order of how often they are the culprit. Oil breakthrough: sebum emerges under the film, lifts it and floats sections of it apart — which is why it starts in the T-zone and shows up mid-afternoon. Incompatible layers: a silicone-based product under or over a water-based one, where the two do not bond and the upper film shears off in patches. Too much product: a thicker film has more weight and less flexibility, so it fractures. And unabsorbed skincare: foundation applied onto moisturizer or sunscreen that was still wet sits on a film of water or oil rather than on skin, so it never adheres properly in the first place.
Where and when it happens tells you which one. Nose and chin, mid-afternoon, with shine: oil. Anywhere, within an hour, in patches with hard edges: incompatible layers. Worst where you applied the most: too much product. Within twenty minutes, over the whole face, after a hurried morning: unabsorbed skincare.
Why mature skin is more prone to it. Less oil overall but still-active glands in the T-zone means a sharper oil gradient across one face. Skin that has lost elasticity gives a less stable surface for a film to sit on. And the layers have usually multiplied — moisturizer, eye cream, sunscreen, primer, foundation, powder — because that is what the advice recommends, and every added interface is another place two films can fail to bond.
