In Julie and Julia, the usually amusing Amy Adam gets stuck with the disastrous role of a contemporary ninny of a blogger — Which genius decided blogging was a cinematic career? — whose boring modern life only serves to annoyingly keep Streep off-screen for half the movie. In general, you don't want to take a role playing a contemporary character in a film with extensive flashbacks to pre-1960s people — modern characters are too casual to make the kind of imposing impression that old time characters can make. But you especially don't want to play opposite Meryl Streep as Julia Child.
Movie stars tend to emerge from tumultuous upbringings. (For example, I don't know how many current stars spent a couple of years living in hippie communes as children.) Streep, in contrast, has always seemed like the supremely professional product of a proper upbringing. This perhaps made her less sympathetic when she was young in a sort of Jack Nicklaus-Peyton Manning way, but she's enjoying the benefits of an improbably long career today.
Streep might even get a 17th Oscar nomination for her middle aged lady fantasy movie "It's Complicated," a kind of Philadelphia Story "comedy of remarriage" for women of a certain age.
Depression-era movies about rich people, like Philadelphia Story, are known as "white telephone movies" because only millionaires could finagle a non-black telephone out of the Bell monopoly back then. Perhaps the contemporary equivalents made by Nancy Meyer (writer director of the aptly named What Women Want with Mel Gibson) could be called Viking range movies because they are heavy on high-end kitchen appliance porn.
The last 60ish leading lady to be on top of the box office was, I'm guessing, 250-pound Marie Dressler, who was born during the Johnson Administration (the Andrew Johnson Administration). Most very early talkies are close to unwatchable, so Dressler is remembered today mostly for 20 seconds with Jean Harlow in 1933's Dinner at Eight.