Conventional wisdom (aka pop culture) suggests that coolness fades as a person gets older. I entirely disagree, but that’s a subject for another thread.
Septuagenarian Adam West (Batman) gets my vote as really cool–the kind of guy I would really enjoy shooting the breeze with over lunch or golf. Paul Newman is an obvious pick, but I’m looking for others.
Your votes?
Ian McKellen seems like he’d be fun to party with if you can keep up. Hugh Downs was going on archaeological digs and swimming with sharks, both just for fun, when he was well over 70 (it was in an interview with him some years ago) and until emphysema and a long neglected body slowed him down Kurt Vonnegut seemed to get new wind after he was 60 as did Studs Terkel, who in interviews in his 80s seemed as pissed off as ever. Helen Thomas, a fixture at every White House press conference since James A. Garfield, is still the wrath of God in her 80s as well.
Sorry- I just realized the OP asks for actors, so I’ll leave the above at Ian McKellen. (80 something Helen Thomas is still cool though.)
Robert DeNiro.
Oh, baby.
And the other perennial gangster movie favorite, Al Pacino.
Damn. Beat me to it. 
Adam
Patrick Stewart, without a doubt. I think I’ve had a crush on him since I was about sixteen. I’d love to have a conversation with him, just to hear that voice in person, but I know I’d be all twitterpated and wouldn’t remember a second of it.
Can I count Sam Elliot, who at least looked like an old fart circa “Roadhouse”?
A very sexy old fart… 
Just off the top of my head, I’d add perenially cool Paul Newman to the list.
Oh, and Stockard Channing, just because I never see her in anything that she isn’t cool in.
Ohhhh, Sam Elliot! Haven’t seen him in years but just thinking about his piercing blue eyes still makes my spine tingle! Oh my!
Now I’m too melty-feeling to think up anyone else…
Clint Eastwood, very cool.
Sean Connery, of course!
Harrison Ford is still a man’s man at 63. The way he doesn’t seem to care about his movies anymore is sort of cool.
Does Willie Nelson count as an actor? He’s been in his share of movies, and he’s still a pot-smoking independent country music rebel at age 72.
Peter O’Toole. That guy is awesome.
That stupid earring and the trophy girlfriend disqualify him.
I’ve said before that the young Peter O’Toole, circa Lawrence of Arabia, makes me wish I was (were?) gay.
jackelope wrote:
I myself am totally gay for Peter Falk.
Paul Newman. Not only is he cool, I, not even 25 years old, would still do him.
Also, Sean Connery, Peter O’Toole (I was hoping he could be the replacement Dumbledore, but perhaps he’s too drunk), I still get hot for Harrison Ford (all that Indy Jones as a kid did me in, I imagine) Raquel Welch (how you get to be 60 and still be unbelievably hot, I would like to know) and Eartha Kitt. I love Eartha Kitt. And Sophia Loren.
And God help me, I’d love to hang with Shatner, Takei, and Nimoy.
I’d do this guy in a heartbeat. Too bad Catherine Ross already bagged him.
That voice. Those eyes…the stories he can tell.