I would say choose the age you were diagnosed, but I myself wasn't ever diagnosed and proper diagnosis can take time. So I guess, while this is a bit skewed, choose the age that you believe you first became/were eating disordered.
Obviously, this doesn't apply to those who aren't eating disordered.
And, if you previously had an ED and since recovered, if you could comment with the age of recovery...
What age did your ED begin?