We assume that aging is the cause of damage on the bodies cells and DNA. This means that eventually you will have damage on "all" your cells in your body after a time period (aging).
Then if so, how come that we can after millions of years, generation after generation reproduce and create healthy offspring? The female egg and the mans sperm would have a lot of damage. This would in theory mean that we would create offspring that is already aged. But that reality isn't so.
So, what I know is that the sperms and eggs are fresh and "free" from DNA damage, so we can create healthy offspring. Then why is the reproduction material sperms and eggs free from aging damage, if the aging theory is mainly about wear and tear?
The first part of the answer was already given by Maestro949. Think of it in these broader terms. All organisms suffer aging damage to their molecular and cellular structures, yet the rate of aging varies from one species to the next, because natural selection has created conditions that select for different levels of investment into mechanisms to prevent and repair that damage. The amount invested in these mechanisms (and thus diverted away from other, also-crucial priorities, like fast growth, sharper claws, greater sprinting speed, etc) is determined by natural selection.
But natural selection does not optimize for traits that benefit the health of the individual organism, but for
fitness -- for traits that increase the ability to leave behind offspring that are, themselves, viable. How much fitness is conferred by more robust defenses against aging damage for a particular organism living in a particular niche depends on how many more viable offspring giving better defenses will confer, granted all the other things that threaten to kill the animal
no matter what its rate of aging, granted its niche. So for instance flying rodents (bats, flying foxes) age more slowly than scurrying ones (mice and rats), because the former find it easier to escape predators, and thus investments in more robust anti-aging machinery pay off in additional viable young, because they don't get killed off before that extra potential for youth (including youthful reproduction) ever comes onto the scene.
So the first part of the answer is that, to the extent that sperm and egg DNA may be better protected against aging damage than the rest of the organism, that's because mutations that have led to such additional protections have paid off in past evolutionary time, for exactly the reason that you mention:we wouldn't be able to reproduce if our gametes' DNA weren't kept in very 'clean' shape.
The second part of the answer is that the 'lifestyle' of the egg in particular to some extent reduces its exposure to aging damage
by default, because they spend most of their time in a deeply quiescent metabolic state (thus being exposed to fewer free radicals and other damaging agents caused by metabolism) and don't themselves divide for much of the time that the rest of the body is aging (unrepaired errors in DNA replication in preparation for cell division are the main cause of mutations, not attack by intrinsic or extrinsic damaging molecules).
But the third part of the answer is that the question itself is to some extent of the 'have you stopped beating your wife yet?' sort -- ie, there's a false premise buried in the question (since (I generously assume) you actually didn't beat your wife in the first place). In fact, sperm and egg DNA (not to mention their cellular structures) do suffer quite a bit of aging damage, which is part of the reason why older parents are less fertile (the sperm and eggs or their DNA are duds), have more miscarriages (in addition to problems in the ability of older mothers' bodies to support the developing fetus, the fetus itself is more likely to have fatal flaws), and are more at risk for birth defects (because of mutations in the egg and sperm DNA):
Birth Defects Genetics Center
PRECONCEPTIONAL COUNSELING
Virginia P. Johnson, M.D. c2000
Advanced maternal age is an indication for amniocentesis because of the risk of Down syndrome and other chromosomal disorders. In females, eggs are present at five months in utero. At birth there are four million eggs frozen in the dictyotene phase of meiosis 1, and after fertilization, these complete meiosis 2. A baby born to a 40-year-old woman literally comes from a 40-year-old egg, which then would have been exposed to environmental agents that can cause nondisjunction or failure of normal separation of chromosomes. This leads to aneuploidy, the most common of which is trisomy 21 [Down syndrome. Note that this is only one of several reasons for the increase in Down syndrome risk in older mothers, whose relative contribution is still undertain -MR]. Advanced paternal age also has an adverse effect. Mutations in a spermatogonium are replicated and passed to spermatocytes. Mutations associated with advanced paternal age include achondroplasia, Apert syndrome, neurofibromatosis, hemophilia, Marfan syndrome, Treacher-Collins syndrome, Waardenburg syndrome, thanatophoric dysplasia, and osteogenesis imperfecta; these are autosomal dominant traits. Examples of X-linked traits, for which a paternal age effect has been implicated include fragile X, hemophilia A (factor VIII), hemophilia B (factor IX), Duchenne muscular dystrophy, incontinentia pigmenti, Hunter syndrome, Burton agammaglobulinemia and retinitis pigmentosa.
Also in this area, it's worth noting that a lot of the reason why eggs are statistically in unexpectedly good shape when they are called to duty is not that they are better maintained, but that they are selected out: the woman's body destroys many defective eggs, to avoid wasting a reproductive opportunity by backing a flawed horse.
-Michael
