A lot of the decline in happiness is related to starting a family. Average happiness is lower with kids (there are much higher peaks because of them, but also lower lows http://www.ted.com/talks/rufus_griscom_alisa_volkman_let_s_t...) because of all the chaos related to kids.

Average happiness sounds absolutely preposterous.

What about it sounds preposterous? Certainly happiness isn't easy to measure, but it seems like asking "On a scale of 1-10, how happy are you with your life?"(or something like it) is a really important question... And as a society, we'd like that number to be broadly high and climbing as we move forward... No?

I mean judging your own happiness against the mean or median of 300 million people who live in vastly different locations and climates, with different population densities, prevalence of poverty and opportunity, availability of social services, access to education and medical services, etc, is a meaningless exercise.

I don't think anyone was comparing happiness against other people here. It's happiness against yourself. Are you happier now than you were last year? Average happiness in this context doesn't mean the average happiness of the world, or a community. It's just how you would deem the average happiness of your life at a certain era. Yesterday I had a terrible day, but I would still say I am happy on average, I just was not happy on that particular day.