Eastwood played an old geezer. Meryl Streep played Margaret Thatcher.
Denzel is playing a badass wrecking machine taking on hoodlums. That's about as believable as Starbuck in BSG being a brawling bruiser taking down buff guys.
He's a vigilante with a weapon, and probably training, so you are wrong as usual.
And between whoopin on drug dealers, he leans on his walker to catch his breath, then goes home and checks to see if his SS check arrived yet.
Good grief! He isn't THAT old!

Besides, a lot of women find older men sexy.
He's a few months shy of 60. Are golden years audiences really crying out for action movies where bodies are piling up or is that kind of movie more appealing to an unsophisticated young man audience. What 20 year old wants to go see a movie about some grandpa kicking ass?
Considering action movies with Liam Neeson, Bruce Willis, Stallone and his ensemble of old action stars, etc. are being made and making money, I'd say there's plenty of audience for older action stars. And of the older ones out there, I think Denzel Washington is probably one of the more believable. His action is often a bit more subdued than some of the others.
On an unrelated note, I think Man on Fire may be my favorite Denzel movie.