Well, legal age is 21 here in the States but I didn't know a single American growing up who waited until then. Even I was straight-laced as they came until I turned 16 and it was all downhill from there.
Honestly, I don't really wish to defend light beer here (I don't mind Coors Light but Bud Light makes me want to gag) but it's not really about "dieting," it just tends not to fill you up as quickly, so you can drink more. But craft beers have exploded here in the US, most anywhere you go whether it's a private party or a bar/restaurant, there's usually at least one type of craft beer to be had. I honestly don't recall the last time I had a Coors out and about.
Now stuff like Michelob Ultra is more about avoiding carbs and all that nonsense, but even then I don't think people GAF about it--the fattening content of beer is alcohol, ain't no one skipping THAT.