What can go up without ever coming down?
Answer: Your age.
As time passes, your age keeps increasing. It never reverses and becomes a smaller number again.
The riddle works because the phrase “go up” usually makes us think of something physically rising, such as a balloon, elevator, or airplane. Those things can also come back down.
Age is different. It “goes up” in a numerical sense rather than a physical one.
Once you recognize that the clue is about increasing rather than literally moving upward, the answer becomes simple: your age.
