Trunks

 Trunks is the son of Bulma - a headstrong, technical genius - and Vegeta - one of the most powerful fighters in the Dragon Ball Z universe. His exact age is not certain, but he first appears in Dragon Ball Z as a teen. He is from the future and has witnessed a threat to Earth which cannot be stopped. He has come back in time to try to save the future Earth, only to find our heroes in Earth facing their own peril.

 Much to the amazement of the Z gang, Trunks immediately turns Super Saiyan and defeats Frieza and King Cold. He gives Goku medicine to prevent his death from heart disease and tells them of Earth's coming predicament. The Z Fighters immediately go into training, and Trunks returns to his own time.

 Trunks returns to our timeline again in the Android Saga to ensure that the Androids do arrive on Earth. He is also born during this saga, so we have the opportunity to see him both as a teenager and a baby at the same time. He continues to exist as a boy and a teen until the end of the Cell Saga, when the older Trunks returns to his original timeline.