An interesting, albeit dumb, perspective
I got an anonymous comment yesterday to my post from earlier this month about Peter Capaldi being cast as the 12th Doctor:
The grandfatherly Doctor belongs to a different time, 50 years ago in fact! Doctor Who is a different show now and its casting of the lead should reflect that. This is such a huge step backwards!
Let's first consider the consider the logical implications of what Anonymus says. It's true that since Doctor Who returned in 2005, the actors playing the Doctor have gotten progressively younger. Christopher Eccleston was 40 when his casting was announced; David Tennant was 34; Matt Smith, 26. If casting an actor older than Smith is a step backward, then logically the show can only step forward from Smith by casting an even younger actor. Tennant was six years younger than Eccleston, and Smith 8 years younger than Tennant; if we take the average, a 19-year-old should have been cast as the 12th Doctor. Or, if we instead look at the arithmatic progression, a 16-year-old. Either way, I'm not convinced it would be an improvement.
(Speaking of averages, just for fun I looked up the ages of the 11 actors who've played the Doctor to date and calculated the age difference between successive pairs of actors. The biggest gap was between Peter Davison and Colin Baker: +12. The smallest was +3, between C. Baker and Sylvester McCoy and between Paul McGann and Eccleston. The average age is 40.3, and the average age difference -3. Factoring in Capaldi, the average age bounces up to 41.5, about what it was before Smith was cast, and the average age difference drops to zero.)
Maybe that's not what Anonymus meant, though. Maybe she means that recent Doctors have been more action-oriented and more romantic than those seen on the original series. (That's romantic as in romantic love. The Doctor has often been a Romantic hero (as in Romanticism) and the show's always been in the tradition of the chivalric romances of the middle ages and Renaissance.) But does casting an older actor preclude the show from continuing in that vein? I think not. I would suggest that the Doctor who most closely resembled the romantic action heroes seen in the new series was the 3rd, played by Jon Pertwee. He was definitely more action-oriented than his predecessors and most of this successors, and many viewers have detected romantic overtones in his relationship with Jo Grant. And how old was Pertwee when he got the job? 50.
All that aside, the one constant throughout the history of Doctor Who is that it's always changing and always has been changing. I don't think any change to the show can be called a step backwards, because change is written into the show's DNA, and that constant change is how it's managed to keep moving forward for 50 years.