  Stellar population models to predict the H Balmer and HeI lines in absorption
are presented. Models are computed for burst and continuous star formation. It
is found that the Balmer and HeI line profiles are sensitive to the age, except
during the first 4 Myr of the evolution, when the equivalent widths of these
lines are constant. The comparison of these lines with the corresponding
nebular emission lines indicates that Hd and the higher-order terms of the
Balmer series and HeI (3819, 4026, 4388 and 4922) in absorption are very good
age-indicators of starbursts. Models are applied to date the super-star cluster
B in NGC 1569.
