Context :

Aims : We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence , approximately solar-type ( G-K dwarf ) stars in time series of 36 years from the Mount Wilson Observatory Ca ii H & K survey .
Our main aim is to search for correlations between cycles , activity measures and ages .

Methods : Time-frequency analysis has been used to discern and reveal patterns and morphology of stellar activity cycles , including multiple and changing cycles , in the datasets .
Both the results from short-term Fourier transform and its refinement using the Choi-Williams distribution , with better frequency resolution , are presented in this study .
Rotational periods of the stars were derived using multi-frequency Fourier analysis .

Results : From the studied 29 stars we found at least one activity cycle on 28 of them .
Twelve stars , with longer rotational periods ( 39.7 \pm 6.0 days ) have simple , smooth cycles , and the rest of the stars , with on-average much faster rotation ( 18.1 \pm 12.2 days ) show complex and sometimes vigorously changing , multiple cycles .
The cycles are longer and quite uniform in the first group ( 9.7 \pm 1.9 years ) , while are generally shorter and with greater variety in the second one ( 7.6 \pm 4.9 ) .
There is a clear age division between stars with smooth and complex cycles that follows the known separation between the older and younger stars at around 2 to 3 Gyr of age .

Conclusions :