  Using optical/near-IR broadband photometry together with Halpha emission line
data, we attempt to constrain the star formation histories, ages, total stellar
masses and stellar mass-to-light ratios for a sample of extremely blue low
surface brightness galaxies. We find that, under standard assumptions about the
stellar initial mass function, the Halpha equivalent widths of these objects
appear inconsistent with recently suggested scenarios including constant or
increasing star formation rates over cosmological time scales. In a critical
assessment of the prospects of obtaining ages from integrated broadband
photometry, we conclude that even with near-IR data, the ages are poorly
constrained and that current observations cannot rule out the possibility that
these objects formed as recently as 1-2 Gyr ago. Methods which could
potentially improve the age estimates are discussed. The stellar masses of
these galaxies are inferred to lie below 10^10 solar masses. This, in
combination with low ages, could constitute a problem for current hierarchical
models of galaxy formation, which predict objects of this mass to form
predominantly early in the history of the universe. The possibility to use the
ages of the bluest low surface brightness galaxies as a test of such models is
demonstrated.
