When I wrote earlier this week that McDonald’s plans to improve the nutritional value of its Happy Meals, I — like most others whose commentaries on the subject I read — focused on the impact of fast food on young children’s diets. I righteously noted that it’s up to parents to decide how many Happy Meals their kids eat and when they eat them.

But what happens when kids grown into teenagers, with money in their pockets and independent access to fast-food restaurants? How does a parent uphold nutrition standards, other than to hope that some of what we taught our toddlers remains lodged in our adolescents’ brains?

The issue came to mind when I read a report from the UCLA Center for Health Policy Research, which found that 58 percent of teens in California had a soda or other sugary beverage at least daily and that 46 percent ate fast food at least twice a week.

The report analyzed the environments surrounding teens’ homes and schools according to the relative availability of healthful food versus less-healthful fast or convenience food. Not surprisingly, kids whose neighborhoods or schools were surrounded by fast-food restaurants, dollar stores, pharmacies, liquor stores and convenience stores consumed more “junk” food than those who had easier access to grocery stores, warehouse stores and produce vendors.

The report looked only at California, but it makes sense that things would play out similarly in other locales.

Looking back, the time in my life when I ate the least-nutritious foods was when I first had some spending money and was allowed to walk to the shopping center across the street from my junior high school. (This was back before we called them middle schools.) I bought candy bars, Slim Jims and other sugary/salty/fatty snacks from the High’s convenience store and People’s drug store there. It took me many years to shift to a more healthful diet.

Among other suggestions, the California report calls for public and private efforts to bring retail outlets offering better-for-you foods to the key areas where teenagers are likely to hang out. Among those outlets, the report lists farmers markets.

To 50-year-old me, that sounds terrific. But I’m wondering how many teenagers will be inclined to drop their allowance on food from a farmer’s market; junior-high me sure wouldn’t have been.

Maybe some years down the line, with some education efforts to encourage such a change, that will start to sound more plausible.