Usually, with statistics, you say i polled 1000 people and most of them were 25-30. You might conclude that most people are that age. But there are self selection artifacts, and maybe the octegenarians don't respond to such polls.

Consider this. There are about 6,000,000,000 people on Earth alive today. Some estimates suggest that over all history, only 36,000,000,000 people ever lived. So 1/6th of all people who have ever lived are still alive. This suggests that a pretty big fraction of people never die. This isn't a statistic from a small sample. This is a sample of the entire population!