This can depend. In the Old Jedi Order, as Jedi were trained from infancy, they constructed their own lightsaber while as a Padawan learner under a Master. Padawans are generally chosen around age thirteen and become Knights in their twenties, so the teenage years for them. The next generation of Skywalker's Jedi, like his niece and nephews Jaina, Jacen, and Anakin, construct theirs when they are fourteen years old.