  (Abridged). Low metallicities, large gas-to-star mass ratios, and blue colors
of most low surface brightness (LSB) galaxies imply that these systems may be
younger than their high surface brightness counterparts. We seek to find
observational signatures that can help to constrain the age of blue LSB
galaxies. We use numerical hydrodynamic modelling to study the long-term (~13
Gyr) dynamical and chemical evolution of blue LSB galaxies adopting a sporadic
scenario for star formation. Our models utilize various rates of star formation
and different shapes of the initial mass function (IMF). We complement
hydrodynamic modelling with population synthesis modelling to produce the
integrated B-V colors and Halpha equivalent widths (EW(Ha)). We find that the
mean oxygen abundances, B-V colors, EW(Ha), and the radial fluctuations in the
oxygen abundance, when considered altogether, can be used to constrain the age
of blue LSB galaxies if some independent knowledge of the IMF is available. Our
modelling strongly suggests the existence of a minimum age for blue LSB
galaxies. Model B-V colors and mean oxygen abundances set a tentative minimum
age at 1.5-3.0 Gyr, whereas model EW(Ha) suggest a larger value of order 5-6
Gyr. The latter value may decrease somewhat, if blue LSB galaxies host IMFs
with a truncated upper mass limit. We found no firm evidence that the age of
blue LSB galaxies is significantly smaller than 13 Gyr.
