Consider first the free radical theory (see this and this). The theory states that free radical produced in mitochondria are responsible for the ageing since they are highly reactive and cause damage for the DNA. One can however wonder what is the mechanism causing the generation of the free radicals.
A TGD based justification for the free radical theory came as unexpected application of the quantum model for how metabolic batteries are loaded in many-sheeted space-time. The kicking of electrons to smaller space-time sheet loads metabolic batteries in TGD Universe. The dropping of electrons back liberates metabolic energy. These processes occur all the time in ADP<-->ATP "Karma's" cycle. In earlier posting I discussed quantitative model for the burning of water producing hydrogen peroxide and hydrogen (this process could provide a mechanism of storing solar energy by a mechanism analogous to photosynthesis).
This kicking also induces 2H2O-->H2O2+H2 as eplained in previous posting. Unfortunately, the kicked electrons induced very probably many process unavoidably producing highly reactive reaction products (for finnish readers this gives new meaning for the saying "Ei elämästä selviä hengissä!"). The generation of these harmful reaction products can be interpreted in terms of second law of thermodynamics: loading process involves some dissipation of energy to unintended purposes and increase of entropy and this leads to ageing. Antioxidants is the means of the organism to minimize these effects. The fortunate aspect is that hydrogen peroxide can be used by the immune system to kill harmful bacteria as Mae-Wan Ho tells in one of the articles (for a list of references including references to Mae-Wan Ho's articles see the earlier posting).
The fortunate outcome is oxygenation of atmosphere, which has played key role in evolution (see the article of Mae-Wan Ho providing a valuable overall view). For instance, the increase of the oxygen fraction from 2 per cent to about 20 per cent is believed to have preceded Cambrian revolution and its further increase to about 40 per cent in maximum is believed to have made possible the emergence of organisms with gigantic size with large metabolic needs (trees, dinosauri and other lizards, and insects). The subsequent reduction of the oxygen fraction down to 15 per cent in minimum has been seen as a reason for the disappearence of large insects and reptiles and also as a reason for long period of stagnation in evolution.
The TGD based model of Expanding Earth explains the strange fact that all continents form a single super-continent if the radius of Earth is scaled down by a factor of 1/2. Expansion is assumed to be due to the occurrence of a quantum phase transition increasing the value of Planck constant by a factor 2 for the space-time sheet of Earth (see also the chapter Quantum Astrophysics" of "Physics in Many-Sheeted Space-time". This led to the proposal that the reduction of gravitational force by a factor 1/4 during the expansion also contributed to the increase of the size. A related proposal is that the underground water reservoirs carrying highly developed life forms were bursted to the surface of Earth in Cambrian explosion. An interesting question is whether the burst of large amounts of underground water to the surface of Earth was the mechanism leading to the oxygenation of the atmosphere. The oxygen could have been present in the interior before the expansion and allowed the evolution of photosynthesis (important progress takes often place underground;-)).
For a more detailed representation or latest progress see the chapter Quantum Model for Nerve Pulse of "TGD and EEG" or the chapter Homeopathy in Many-Sheeted Space-Time of Bio-Systems as Conscious Holograms.