Obviously being 3 years older then Cavil is right now, 4 years older then Christopher Reeves was in his last Superman movie, 1 year older then George Reeves was when his show
debuted and 1 year older then Kirk Alyn was when he became the first live action Superman makes Brandon Routh an absolute fossil
Comparing him to other (non Superman) heroes, Routh is:
4 years younger then RDJ was when Iron Man 1 came out
1 Year older then Michael Keaton was in the 1989 Batman
2 Years older then Val Kilmer was in Batman Forever
3 Years older then George Clooney in Batman & Robin
5 Years Younger then Ben Affleck in Batman v Superman
The same age Jason Mamoa was in Aquaman
Varying years younger then these members of Suicide Squad's cast: Will Smith, Jared Leto, Jay hernandez (and he's the same age as Joel Kinnaman)
1 Year older then Zachary Levi (from Shazam) is right now
7 Years younger then Paul Rudd was in Ant-Man 1
Also, fun fact, if the Nic Cage Superman film had been made, probably coming out in the very late 90s or early 2000s based on when they were working on it (the script being done around 1996ish), Cage would have been probably between 4 to 1 years younger then Routh is right now. But, yeah, Routh is too old
To be clear, there is no chance of Routh playing Superman in a film at this point, but his age really isn't the main factor, or probably even on the list of major reasons that will never happen.