Ageing skin is usually described in terms of wrinkles, which are the visible consequence of a structural change occurring in the layer beneath the surface.
Structure sits below the visible layer
The outermost layer is a thin barrier of flattened cells providing protection and water retention. Beneath it lies the dermis, which supplies mechanical structure.
The dermis is largely a matrix of collagen fibres giving tensile strength, elastin fibres allowing recoil, and water-binding molecules keeping the whole structure plump.
Its thickness and organisation determine how the surface sits, which is why changes below produce effects visible above.
Production and breakdown fall out of balance
Collagen is continuously produced by dermal cells and broken down by enzymes, with the balance determining net content, in the same pattern as bone.
Production declines gradually across adulthood while breakdown enzyme activity does not fall correspondingly, so net collagen decreases year on year.
Elastin is worse placed still, since it is produced mainly early in life and is largely not replaced once damaged, which is why recoil is lost permanently.
Ultraviolet exposure accelerates the process
Ultraviolet radiation both damages collagen directly and increases production of the enzymes that break it down, attacking the balance from both sides.
Comparing skin on habitually exposed and habitually covered areas of the same person shows the size of this effect, since genetics and age are held constant.
This is why sun protection is the intervention with the clearest evidence in skin ageing, and why it is preventive rather than corrective.
Thinning changes surface behaviour
As the dermis thins, the junction between it and the layer above flattens, reducing the surface area over which nutrients pass and the two layers grip each other.
Skin then becomes more fragile, bruises more readily and heals more slowly, effects that matter more clinically than the appearance of lines.
Reduced oil production and slower barrier repair also make dryness more common, which is a separate change with a separate cause.
What topical products can reach
Most molecules applied to skin do not cross the barrier layer in meaningful quantity, which limits what a cream can do to the dermis below.
Collagen applied topically is far too large to penetrate, so a product containing it acts on the surface rather than replacing what has been lost.
Ingredients with dermal effects work by signalling to cells rather than supplying material, and anything producing real structural change is a matter for a dermatologist.