Notice that this changes right around the age of puberty. Having grown up quite poor, my hypothesis is straightforward.
Before you are a teenager, the things that make you happy and that you value are largely disconnected from money. Happiness is more about the environment not being pathological more than it is about having material goods. Children invent their own happiness.
As a teenager, it becomes about the mating ritual. In that context, relative material deprivation can have a significant adverse impact on the ability to attract a quality mate. I would expect that in countries where it is relatively easy to change your material position in society would have happier adults than one where there was less opportunity because it gives the person more ability to improve their outcomes on the mate selection front.
Before you are a teenager, the things that make you happy and that you value are largely disconnected from money. Happiness is more about the environment not being pathological more than it is about having material goods. Children invent their own happiness.
As a teenager, it becomes about the mating ritual. In that context, relative material deprivation can have a significant adverse impact on the ability to attract a quality mate. I would expect that in countries where it is relatively easy to change your material position in society would have happier adults than one where there was less opportunity because it gives the person more ability to improve their outcomes on the mate selection front.