“Nobody grows old merely by living a number of years. We grow old by deserting our ideals. Years may wrinkle the skin, but to give up enthusiasm wrinkles the soul.” -Samuel Ullman
When it comes to the Universe, there are some dead giveaways as to what its age is. Its elemental composition changes, the types of stars that are present evolve, the large-scale structure visible to us morphs, grows and ceases, and the temperature of the cosmic microwave background drops, among many other signs.
Yet when we put them all together, there are only two methods available to measure the age of the Universe: the measurement of its expansion history and the measurement of the age of the oldest stars. The first is by far the more accurate, at 13.81 billion years (plus or minus just 120 million), while the second validates that picture, with a maximum age of 13-to-14 billion years.