Intrinsic causes of skin aging or skin aging theories are divided into 2 categories:
1) aging due to the accumulation of molecular or cellular damages
This group of aging theories include: free radical theory of skin aging, inflammaging, cross-linking of bio-macromolecules by glycation, mitochondria damage, DNA damage, and others.
2) aging due to the genetic programming
Not only aging theories from the same category are interrelated, damage based theories and programmed theories are also interrelated.