Since Bradley Cooper just turned 40 on Jan/5 we could include those 40 and under who are in the Top Tier of working actors.
Who else needs to be on such a list? (An IMDb link would be nice.) 
Leonardo DiCaprio is definitely A-list and he is barely 40.
Jennifer Lawrence
Anne Hathaway
Jessica Chastain
Benedict Cumberbatch is just under the wire.
I was gonna say JLaw. Chris Hemsworth?
While we’re at it, the other two Marvel Chrises, Evans and Pratt, are also under 40. I don’t know if Evans is quite A-list, but Pratt starred in not one but two of the most profitable movies of 2014, which seems like a pretty solid qualification.
Scarlett Johansson should be considered to be an A list actress.
Emma Stone is only 26. I thought she was closer to 30. Anyway, she’s an A lister.
No. Not ever.
Only if the category is “Best performance by a forest product.”
Which I guess raises the question: Are we looking for a list of the most talented actors, or those with big drawing power? Define “A List.”
What do we consider A list? Tom Cruise - sized paychecks? Lead in at least one successful big budget movie?
In 2014 the biggest grossing stars under 40, according to Forbes, were Jennifer Lawrence (24), Chris Pratt (35), Scarlett Johansson (30), Chris Evans (33), Emma Stone (26), James McAvoy (35)-no, really, apparently the latest X-Men did very well, which also explains-- Michael Fassbender (37)
Reese Witherspoon is 38.
Perhaps modifiable to “Best performance by a forest product with nice limbs.” 
And as for definitions of “A List,” just do your own thing. This ain’t for college credit, anyway.
Okay, an attempt to do this scientifically. I went to Box Office Mojo and found the stars under 40 whose movies have earned a total of at least a billion dollars.
Drew Barrymore
Justin Bartha
Jay Baruchel
Orlando Bloom
Russell Brand
Abigail Breslin (the youngest in this list at 18)
Rose Byrne
Benedict Cumberbatch
Kirsten Dunst
Chris Evans
Dakota Fanning
Anna Faris
Colin Farrell
Isla Fisher
Megan Fox
James Franco
Tyrese Gibson
Joseph Gordon-Levitt
Ashley Greene
Rupert Grint
Naomie Harris
Anne Hathaway
Chris Hemsworth
Liam Hemsworth
Tom Hiddleston
Jonah Hill
Josh Hutcherson
Scarlett Johansson
Angelina Jolie
Keira Knightley
Shia LaBeouf
Taylor Lautner
Jennifer Lawrence
Justin Long
Kellan Lutz
Tobey Maguire
Jena Malone
Rachel McAdams
Robert Pattinson
Chris Pine
Natalie Portman
Daniel Radcliffe
Jackson Rathbone
Ryan Reynolds
Michelle Rodriguez
Seth Rogen
Zoe Saldana
Kristen Stewart
Emma Stone
Channing Tatum
Charlize Theron
Liv Tyler
Emma Watson
Kate Winslet
Reese Witherspoon
Elijah Wood
Sam Worthington
A-list is typically meant to represent one’s power in the industry, as well as one’s name recognition. So I would say Scarlet is definitely in, but Christ Pratt is on the cusp–one good year does not an A-lister make. Jurassic World will probably give him that necessary boost.
Similarly, Chastain is incredibly talented, but definitely not an A-lister. Cumberbatch I would argue not quite, because he’s mostly known for indie dramas and imports. His biggest movie is where he voices a dragon.
Stone is on the path but still is known for supporting parts. She still needs to carry a big movie (and no, Easy A doesn’t count). Similarly, Chris Evans is too tied to a franchise the way A-listers aren’t. He needs to be in something big outside the Marvel universe.
Righteous list! 10-4 on the scientific approach.
Of the two, I’d say Evans has a much stronger claim to being A-list than Pratt.
Pratt had a great year in 2014. But one of those was a voice acting role. Which means his entire career is based on being the lead in one movie.
Evans has played Captain America in three movies now. Plus he’s done other movies like Snowpiercer, Fantastic Four, Scott Pilgrim, Push, The Losers, Street Kings, Cellular, Not Another Teen Movie. Take away Captain America and he’s still had a solid career.
Take 2014 away from Pratt and he’s never been the lead in a movie. His biggest role before last year was fourth credit in Moneyball.
Interesting list. Thanks for the research!
Now obviously, these aren’t all A-listers. Some are piggy backing on a few successful franchises (Sam Worthington, Naomie Harris, Rupert Grint, Megan Fox) or benefit from animated hits (Jay Baruchel). Predictably, the names I didn’t recognize were all Twilighters, so they don’t count either (at least the two leads from that franchise have broken out a little creatively). Even the Harry Potter and Lord of the Rings films (both very respectable) put the thumb on the scale a bit. Would anyone consider Liv Tyler A-list, rather than just famous? Ditto Daniel Radcliffe.
In most of these cases, being part of a huge blockbuster has allowed them to find a solid footing in the industry. But little of their fame is related to much outside that sphere. Not that that can’t change, but I’d argue it doesn’t elevate them to A-list. Yet.
One part of being an A List is when your name is a household name and very one knows your face. Pratt and Evans are not A List, they are newcomers. Having success in one year is not enough. That is akin to say just because an artist scored a number 1 hit this year is king/queen of pop. It takes while and that takes staying power.
Leonardo DiCaprio is A list, been in the game for so long and has staying power. I would say it takes a minimum of five years to be established as an A List. Dare I say Jennifer Lawrence is still not A List, but she is getting there.
Some on that list are A List but plenty are not.