  Asteroseismological determinations of stellar ages have shown that old
main-sequence dwarfs do not obey gyrochronology. Their rotation is slow
compared to young stars but faster than gyrochronology predicts. This can be
explained by the presence of a maximum rotation period beyond which the
large-scale dynamo switches off and stops providing global magnetic fields
necessary for stellar spindown. Assuming this explanation, the excess of
stellar dynamo parameters over their marginal values can be estimated for given
spectral type and rotation rate. The estimation gives the dynamo number for the
Sun about 10% above its critical value. The corresponding dynamo model provides
- though with some further tuning - reasonable results for the Sun. Following
the same approach, the differential rotation and marginal dynamo modes are
computed for stars between 0.7 and 1.2 solar masses. With increasing stellar
mass, the differential rotation and the ratio of toroidal-to-poloidal field are
predicted to increase while the field topology changes from dipolar to mixed
quadrupolar-dipolar parity.
