  Context. The first interstellar objects, such as 'Oumuamua, Borisov and IM1,
were discovered over the past decade. Aims. We follow the trajectories of known
interstellar objects in the gravitational potential of the Milky Way galaxy to
constrain their possible origin. Methods. We initiate the trajectories based on
the measured velocities of the interstellar objects relative to the Local
Standard of Rest. Since the scale-height of stars in the Milky-Way disk
increases with age, we use the vertical excursion of each interstellar object
from the Milky-Way disk mid-plane to constrain their likely age. Results. The
small vertical extent of 'Oumuamua's past trajectory suggests that it
originated near the mid-plane of the thin disk, implying a likely age younger
than 1-2 Gyr. The maximal excursion of the comet Borisov is similar to that of
the Sun, suggesting a similar age. The meteor IM1 exhibits yet larger vertical
excursions, suggesting an older source. Finally, we show that human-made
interstellar probes, like Voyager 1 or Pioneer 10 will arrive at the opposite
side of the Milky Way disk relative to the Sun in $\sim$ 2 Gyr and return to
the vicinity of the Sun before it becomes a red giant.
