Nail hit squarely on the head.
I particularly agree with the statement about "accompanied by an adult" - once kids are into their teens, they don't generally want to go everywhere and do everything with their parents in tow; they're finding their feet as independent human beings, so encouraging them to start (or continue) coming to speedway with their mates instead of mum and dad seems a sensible thing to do.
I stopped attending speedway very soon after I flew the nest and stayed away for nearly thirty years, but football, which I have always attended with friends rather than family has kept me hooked since the age of eleven.
Everyone has their own unique experience, but promoters need to start looking beyond the "speedway is a family sport" mantra and realise that there is more to modern society than a series of nuclear families consisting of a mum, a dad, and two kids who remain eight or nine years of age for ever.
There is no "one size fits all" solution; each track needs to assess the demographics of its own locality, see who's missing (young, old, black, white, gay, straight, men, women, whatever) and start to construct a marketing strategy accordingly.