I could for instance present a quite different model; The notion of "speed of time" is something that is evaluated in hindsight, not while events unfold. A day in a childs life consists of a relatively much larger part of its total experience than for en elderly person. Hence the impression.
The neurological pathways in a childs brain are to a much larger part untrodden while experience in an older brain tends to glide along already well established patterns - no new impressions of significance formed during the processing of sensory input.
See the point?