HD 140283 and HD 19445 were the first stars shown to be metal-deficient (Chamberlain & Aller 1951). HD 140283 is a subgiant lying near the main-sequence turnoff, and thus its position in the HR diagram provides a strong constraint on the ages of the oldest stars in the Milky Way halo. In a program led by PI D. VandenBerg, we have used the Fine Guidance System on the Hubble Space Telescope to measure the trigonometric parallax of HD 140283 and two other extremely metal-poor halo subgiants. We will present our results and the implications for the age of the halo.