Who's happier: 40-year-olds or 70-year-olds?

My experience with people older than me is they tend to give me the once over and with a far-off look in their eye, tell me, 'those were the days, when I was young like you'. I visit my great aunt in a nursing home on a weekly basis and encounter many a wheel-chair bound person looking glancing at me with envy, jealous that I can come and go as I please and navigated the hallways and life with the ease of youth and good health. But happiness and youthfulness don't go hand in hand as the following study shows.

According to recent findings, more people in their 70s claim to be happy with their lives than those in their 40s or 60s. On average, 89% of those in their 70s said they were happy, compared to 87% of those in their 60s and 78% of those in their 40s. Why do you think this is? If I had to guess, I'd say that retirement plays a big role -- people in their 70s finally have a chance to slow down and enjoy life after years of hard work. But I'm sure there's more to than that. What do you think?