In addition to the habit-forming age (if cigarette users are aged 16-60, rational advertising would target 16, not 60), a point is that you don't show your target audience as they are, but you show your target audience as they wish to be.

If you target average 65 year old women, then you show gray-haired above-average-health 55-year women.

If you target average 45 year old men, then you show slim (but not too slim) fit (but not too fit) 38 year old men - that might plausibly be average 45 year old men, but are very much skewed towards the idealized goal.

If you target very overweight people, then you show people that are visibly overweight, but particularly good looking for that weight.

That's what works best, that's how homo sapiens are most receptive to be influenced.

I'm 36 and I've already decided on my brand of toothpaste, toilet paper detergents years ago. I will not buy another brand as long as these brand exists with the same price and quality level. I would be a very bad target for advertising.

Just going from my understanding, which could be wrong... Many people find a household goods brand they like and stick with it. For example, once you decide that Crest toothpaste is good enough for you, you'll likely continue buying it for many years without ever thinking twice about it.

So it makes sense for these household goods brands to target young adults, to become their "go-to" choice for decades to come.