You can be any age, as long as you can reach the eyepiece - reflecting telescopes in particular can hoist their eyepieces up in the air, so you might need a chair to stand on. Some aspects of telescope use, such as the distance you need to hold your eye from the eyepiece for the best image and the need to stay up past bedtime, may be lost on younger children, and older ones may better appreciate and understand the sights on offer, but you’re never too young to begin your journey in astronomy.