The age increases back from the hotspot (Myers & Spangler 1985; Alexander & Leahy 1987), so that a line of sight through a source not exactly in the plane of the sky will intercept regions of different ages. Also, the derived lobe-hotspot separation velocities are substantial, particularly in powerful FRII sources (Liu, Pooley & Riley 1992), so that the back of a lobe will be younger than the front by the time photons take to traverse the lobe. This effect can be estimated from the data, as the difference between the spectra from the front and back of the source can be related to the difference between spectra at different points along the source axis, with due allowance for projection effects. In addition, the age at a given distance from the hotspot might depend on distance from the source axis, giving a spread of ages along the line of sight even for sources in the plane of the sky. Also, the ages might be influenced by changing field strength along the source axis (Wiita & Gopal-Krishna 1990). All these effects will smear out any break in the spectrum even more.
Although it would be difficult to do in practice, there should be differences between the spectra at the edges of a source and in the centre of the lobes (Carilli et al. 1991 looked for this effect in Cygnus A and failed to find it). These differences depend on source orientation, and so should be more pronounced for quasars if the standard unification scheme is correct (Orr & Browne 1982; Barthel 1989). In addition, in an inclined source the spectral break in the back lobe should be more smeared out, because in the front lobe the time delay from front to back of the source partially compensates for the different distances back from the hotspot.