In terms of young drivers, then, the magic number comes from national and company-specific statistics that say people under the age of 25 are more likely to get into accidents. This isn't an opinion -- insurance companies make these assessments based on the facts before them. For example, young men are more likely to get into accidents than women, and married folks are less likely to be in accidents than their single peers [source: State Farm]. So, a married woman at 25 is already likely to have a lower insurance rate than an unmarried male counterpart, regardless of how these two hypothetical people act in real life. After all, if you're more reckless than the company thinks you'll be, they'll be raising your premiums soon enough.
When we talk about the magic number, we're talking about an "apples to apples" comparison, which is to say that a person who is paying a given premium is, on average, likely to pay a lower rate after this age -- and in fact, a good driving record means your rates will continue to go down until you hit 75. But there are plenty of factors that can throw this number off. For example, if you don't get your license as a teenager, or wait until you're 25, you won't really see a dramatic difference in rates, because part of the equation insurance companies use includes your experience on the road.
Of course, the kind of car you're insuring has a big impact on your rates. So, you also get into a higher premium area if you upgrade to a more expensive car. If you get nicer wheels right around the age of 25, your rate may not change much, because the car will be more expensive to repair or replace if it's in an accident, and it could be deemed more likely to be stolen than your previous ride.
Likewise, geography has a big part to play: States with more roads are more likely to have higher premiums, for example, as are areas with higher concentrations of people. In 2002, for example, New Jersey had the highest average rates -- more than twice what consumers were paying in North Dakota, the cheapest state for insurance -- because of the sheer density of traffic [source: NAIC].