It is a common believe that an elementary particle (i.e. lepton and quark) has no constituents. But it is not true.
The essential point is that the constituents do not have any mass. That is the difference. The physicists who have investigated the particles have alway assumed that the constituents, if there are any, have mass. They have bombarded particles (e.g. the electron) with high energy particles to decompose them. That did not happen. So, the conclusion was that there are no constituents. But a particle which is constructed by mass-less constituents can never be broken up in this way. Even if one of the constituents is accelerated in shortest time to c, the other constituent can follow immediately.
I have discussed this point with physicists (professors) who have conducted such type of experiment. They have admitted that there is no conflict between the assumption above and this model.
And look: These particles have an angular
momentum (spin) and a magnetic moment. This can be easily understood if an appropriate size is assumed. The magnetic moment of the electron can be determined with an accuracy of 0.001 by a classical calculation!
For more details please look into
http://www.ag-physics.org/electron