  The distribution of young stars into OB associations has long been in need of
updating. High-precision {\sl Gaia} early Data Release 3 astrometry, coupled
with modern machine-learning methods, allows this to be done. We have compiled
a well-defined sample which includes OB stars and young open clusters, in total
comprising about 47,700 objects. To break the sample down into groupings
resembling associations, we applied the HDBSCAN$^{*}$ clustering algorithm. We
used a Monte Carlo method to estimate the kinematic ages of the resulting
clusters and the Student's $t$-test to assess the significance of the linear
correlations between proper motions and coordinates, indicating the presence of
possible cluster expansion signatures. The ages of the majority of clusters
demonstrating a general expansion at a 1$\sigma$ confidence level are several
tens of Myr, which is in agreement with the expected ages of OB associations.
We found 32 open clusters which turned out to be members of the resulting
groupings; their ages are consistent with one another within the uncertainties.
Comparison of the clusters thus obtained with the historical composition of OB
associations in the literature shows a correspondence between their positions
in the Galaxy but an apparent absence of good one-to-one stellar matches.
Therefore, we suggest that the historical composition of OB associations needs
to be revised.
