It may not be a popular perception
It is, because intuitively it ought to make sense. Bigger is generally going to be better, all other things being equal. Yes, i know they aren't, but tat's why it's a popular perception.
but the best marketing plans are rather clever & people don’t always know when they are being targeted. We are generally heavily suggestible, a lot of safety features are legally mandated so why not make a virtue of it?
Of course. Just as Volvo did in the 70s and 80s, and good luck to them. As I said earlier, Volvos genuinely were safer than the competition, and rightly enough they put it in their ads. Why wouldn't you?
Mercedes claim to have a 20 year marketing cycle, sounds absurd
Not at all. Brand management is a skill, and it occasionally falls over. Burberry were the top of the top, the Queen's tailor, for years. Then they cashed in and cheaped out with scarves and (ugh) baseball caps in the 90s. Marketing disaster. What were they thinking of?
but how many prestige marques make their first impression on children? Quite a few I reckon.
All, one way and another. When I was a kid it was Rolls Royce and Jaguar, the German marques had not yet made an impression. Nowadays of course they have. Kids are exposed to clothes, cars, jewellery every time they turn on the TV, of course they swallow the marketing guff because children don't have a critical eye. They're told something, it becomes a fact. I well remember sitting next to my friend's son, aged about 4, as he watched TV. It hit the ads and they were advertising toys. "I want that", he said. Next ad, the same, and the one after. Everything on every ad, "I want that". "I want that." I'm sure that you do. That's the intention.
What is interesting is that the extremely expensive marques choose where they advertise. You will never see a Rolls Royce TV ad. It would be commercial suicide. Instead they advertise at yacht shows and other events that attract the staggeringly rich as opposed to merely the well-off. They want RR to sell alongside Patek, JlC and Blancpain, and absolutely not to the kind of scratters who would buy a Hugo Boss, TAG or Longines.