  The characterization of the growing number of newly discovered exoplanets
---nature, internal structure, formation and evolution--- strongly relies on
the properties of their host-star, i.e. its mass, radius and age. These latter
can be inferred from stellar evolution models constrained by the observed
global parameters of the host-star --- effective temperature, photospheric
chemical composition, surface gravity and/or luminosity--- and by its mean
density inferred from the transit analysis. Additional constraints for the
models can be provided by asteroseismic observations of the host-star. The
precision and accuracy on the age, mass and radius not only depend on the
quality and number of available observations of the host-star but also on our
ability to model it properly. Stellar models are still based on a number of
approximations, they rely on physical inputs and data that can be uncertain and
do not treat correctly all the physical processes that can be at work inside a
star. We focus here on the determination of the age of HD 17156, an oscillating
star hosting an exoplanet. We examine the dispersion of the age values obtained
by different methods ---empirical or model-dependent--- and the different
sources of errors ---observational or theoretical--- that intervene in the age
determination based on stellar models.
