An igneous intrusion is younger than the rocks in which it is found because it is formed from molten rock that has intruded into pre-existing rocks and solidified. The process of igneous intrusion occurs when magma (molten rock beneath the Earth's surface) is injected into existing rocks and cools and solidifies within them. During this process, the magma may come into contact with the surrounding rock, heating and melting it. The new igneous rock is younger than the surrounding rock because it was formed after the surrounding rock had already solidified.
In other words, the age of the surrounding rock represents the time when it was originally formed, whereas the age of the igneous intrusion represents the time when the magma solidified within it. This relationship between the age of the intrusion and the surrounding rocks is used in geologic dating techniques to determine the relative age of rocks and the timing of geological events.