  Abundances of N, Si, S, and Fe for 45 damped Lyman alpha systems (DLAs) have
been compiled and detailed one-zone chemical evolution models have been
constructed for 30 of them. Assuming continuous star formation, we found that
final abundances in each object can be modelled by adjusting only two
parameters, i.e. its time-averaged star formation efficiency and evolutionary
age, with ranges in our sample of 0.01-1.5 Gyr^-1 and 0.18-2.0 Gyr,
respectively. In addition, average star formation efficiency and evolutionary
age appear to be anticorrelated for the sample, suggesting that the star
formation efficiency in a typical DLA decreases with age. At the same time,
N/Si in DLAs is directly linked to an object's age. There is an apparent
bimodality in the distribution of N/Si values which could be the result of a
statistical accident or an effect produced by a truncated or flattened IMF. We
find that the mean and small dispersion of Si/Fe values is related to the
generally young ages of DLAs, wherein not all Fe has yet been expelled by Type
Ia supernovae. Finally, the large scatter and generally lower values of N/Si of
DLAs with respect to blue compact galaxies, despite their partially overlapping
metallicities, indicate that DLAs are generally younger than the latter.
