  We present a transport model that describes the orbital diffusion of
asteroids in chaotic regions of the 3-D space of proper elements. Our goal is
to use a simple random-walk model to study the evolution and derive accurate
age estimates for dynamically complex asteroid families. To this purpose, we
first compute local diffusion coefficients, which characterize chaotic
diffusion in proper eccentricity (e_p) and inclination (I_p), in a selected
phase-space region. Then, a Monte-Carlo-type code is constructed and used to
track the evolution of random walkers (i.e. asteroids), by coupling diffusion
in (e_p,I_p) with a drift in proper semi-major axis (a_p) induced by the
Yarkovsky/YORP thermal effects. We validate our model by applying it to the
family of (490) Veritas, for which we recover previous estimates of its age
(~8.3 Myr). Moreover, we show that the spreading of chaotic family members in
proper elements space is well reproduced in our random-walk simulations.
Finally, we apply our model to the family of (3556) Lixiaohua, which is much
older than Veritas and thus much more affected by thermal forces. We find the
age of the Lixiaohua family to be 155+/-36 Myr.
