There are many ways to answer this question, as there are many types of stars. The youngest stars are generally those that have just formed, within the last few million years or so. These young stars are often still embedded in their natal clouds of gas and dust from which they formed. Other types of young stars include pre-main sequence stars, which are still contracting and gaining energy as they approach the main sequence on the Hertzsprung-Russell diagram, and post-main sequence stars, which have already left the main sequence and begun to evolve into red giants or other stellar types.
