How old are meteorites?
Most meteorites formed about 4.56 billion years ago, at the birth of the Solar System, making them older than any rock on Earth. Some, such as many Martian meteorites, are much younger.
When people ask how old a meteorite is, they might mean several different things, so it helps to separate three ages. The first is the formation age: when the rock itself crystallised or assembled. For the great majority of meteorites, the chondrites and most asteroid fragments, this is about 4.56 billion years. The oldest solids of all, calcium-aluminium-rich inclusions found in some carbonaceous chondrites, have been dated to about 4.567 billion years, and that figure is used as the starting point of Solar System history. Earth's oldest surviving rocks are around 4 billion years old, because our restless planet constantly recycles its crust, so meteorites preserve a much earlier chapter.
Rocks from larger worlds can be younger. Lunar meteorites mostly formed between about 3 and 4.4 billion years ago. Martian meteorites span a wider range: the shergottites crystallised from lava only a few hundred million years ago, the nakhlites about 1.3 billion years ago, and ALH 84001 roughly 4 billion years ago.
The second age is the cosmic-ray exposure age, which records how long a small fragment drifted through space after breaking off its parent body, usually millions to tens of millions of years for stony meteorites. The third is the terrestrial age: how long it has been on Earth. Recent falls such as Winchcombe have been here only a few years, while some meteorites recovered from Antarctic ice have lain there for hundreds of thousands of years or more.
Exceptional cases stretch further still. In Swedish limestone, geologists have found fossil meteorites, completely replaced by Earth minerals, that fell around 470 million years ago.