  The thermal properties of the surfaces of asteroids determine the magnitude
of the drift rate cause by the Yarkovsky force. In the general case of Main
Belt asteroids, the Yarkovsky force is indirectly proportional to the thermal
inertia, $\Gamma$. Following the proposed relationship between $\Gamma$ and
asteroid diameter $D$, we find that asteroids' Yarkovsky drift rates might have
a more complex size dependence than previous thought, leading to a curved
family V-shape boundary in semi-major axis, a, vs. 1/$D$ space. This implies
that asteroids are drifting faster at larger sizes than previously considered
decreasing on average the known ages of asteroid families. The V-Shape
curvature is determined for $>$25 families located throughout the Main Belt to
quantify the Yarkovsky size-dependent drift rate. We find that there is no
correlation between family age and V-shape curvature. In addition, the V-shape
curvature decreases for asteroid families with larger heliocentric distances
suggesting that the relationship between $\Gamma$ and $D$ is weaker in the
outer MB possibly due to homogenous surface roughness among family members.
