  We demonstrate that the clustering statistics and the corresponding phase
transition to non-equilibrium clustering found in many experiments and
simulation studies with self-propelled particles (SPPs) with alignment can be
obtained from a simple kinetic model. The key elements of this approach are the
scaling of the cluster cross-section with the cluster mass -- characterized by
an exponent $\alpha$ -- and the scaling of the cluster perimeter with the
cluster mass -- described by an exponent $\beta$. The analysis of the kinetic
approach reveals that the SPPs exhibit two phases: i) an individual phase,
where the cluster size distribution (CSD) is dominated by an exponential tail
that defines a characteristic cluster size, and ii) a collective phase
characterized by the presence of non-monotonic CSD with a local maximum at
large cluster sizes. At the transition between these two phases the CSD is well
described by a power-law with a critical exponent $\gamma$, which is a function
of $\alpha$ and $\beta$ only. The critical exponent is found to be in the range
$0.8 < \gamma < 1.5$ in line with observations in experiments and simulations.
