Can't tell the modern, baby-faced actor/tresses apart?  Richard Rushfield probably can, but he's still not impressed (go to the link for some then/now photos): (from Kaus)

Our young actors today, even the finest of them, are just too much of lightweights to carry off historical roles. We are not capable anymore of portraying a time when people didn’t act like pouty teens well into their 50’s.

To be fair, modern science as well as modern mores are to blame.  It's easy to develop a rugged, mature look when every restaurant, workplace, and home is wreathed in cigarette smoke; standard fare is meat and potatoes; and the typical exercise regimine consists of lighting cigarettes and eating meat and potatoes--and maybe some golf.

If you made Cary Grant and Jimmy Stewart give up the smokes and put them on a vegan diet they'd still look pretty good.