Why do genetic diseases sometimes only develop in later life?

Although the gene could be turned on from birth and doing damage all the time, it may take a long time for the damage to cause a major problem. For example, if you have a gene for a blood disease that creates something to block up blood vessels, it could take fifty years before it has accumulated enough to actually block them. There is another disease called Huntingdon's disease, where the gene is active from birth, and produces a protein that builds up in a cell. Like a rubbish bin, you can keep adding rubbish until eventually it overflows and causes a problem, so the disease only hits when you get to 40 or 50 years old.