Relationship between happiness and age.
IMPORTANT: Do not compare one graph with the other, trend is relative to the country.  
Fascinating data. Relatively speaking (comparing the edges) I'm surprised US teens are not happier. Sadly the really good stuff is past the average life expectancy!  

My hypothesis is that if you live beyond 75, you are likely in better health, more financially well off and more likely to have that happiness upswing.

China and Lat Am (which is not a country if you are curious) are interesting. I wonder if that is because of the nuclear family structures were parents and grand parents all stay together and kids have deep responsibilities for parents, unlike in the western world. In India that is true as well, family responsibility carried by the kids.

Of course the most depressing graph (again, don't compare countries) is Russia. It starts very optimistic and then everything simply goes downhill.

The usual caveats about survey data, perhaps sampling bias etc apply.  Even with all that, fascinating data and food for thought. Interesting to reflect on your own life and consider how you would draw the curve/straight line.

PS: What is your theory about what is going on in the UK? I've not lived there. 

Data source, methodology etc:
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