I'm no doctor, but it at least seems plausible from a lay-man's perspective.
Immunity is highly specific. This is why the flu is a chronic problem, it mutates rapidly enough that exposure to previous generations of the virus does not confer immunity to current generations. Immunity is more like a collection of solutions to puzzles than it is a muscle.
Being infected while in a healthier state and thus gaining immunity with less risk might be beneficial later, but you can look at mortality statistics and see that while infections are a major cause of death, they aren't responsible for a large portion of deaths:
http://www.cdc.gov/nchs/fastats/deaths.htm
(Chronic lower respiratory disease there is stuff like COPD, not particularly infection related)
That's a fair point, I'll admit I was mistaken.
* As an adult
If you take public trans all the time, then you're doing a lot of walking. Trans takes you close enough but the 'last mile' is always on foot. That's a decent amount of exercise, at least compared to someone who is always driving. This is how I grew up and when I was older, the move to the suburbs was pretty jarring. Everyone got fatter as we stopped getting all that city walking. Worse, the suburbs didn't have a lot of healthy food options. So you drove more and ate crappier.
A pack of cigarettes in NYC is around $13. In a poor rural or suburban community, its half that. A lot of the urban poor simply can't afford smoking anymore. This must certainly affect things, at least as much as anti-smoking PSAs.
NYC also has 1.1m Jews, who tend to have a slightly higher than average life expectancy, especially the religiously observant who follow strict dietary limits. That might skew things a little as well.
I thought it was 500,000 Jewish persons but it looks like 1.1m checks out from a quick google search. That's a huge percentage of the population.