I found one in my late 40s and one in my early 50s. That's the definition of anecdotal evidence, but there it is, for what it's worth.
What do you need an agent for? I've found that they are nothing but gate keepers for a dying industry. There is more money and publishing opportunities with the world's largest publisher - Amazon. I believe the days of traditional publishing for crime fiction is almost gone, limited now to established stars, promotional books for upcoming movies, and those who go to cocktail parties in New York.
You'll know I'm right when NO agents respond to say I'm wrong. The only real hope for newer writers is ebooks. And that is very exciting because when the only game in town was traditional publishing, we didn't have a chance.
To be fair I've never seen an agent respond here to anything. (I have seen an editor though.)
It's true one doesn't need an agent to get one's work in front of the public anymore, but I don't think the agenting industry is dying out, only shrinking and morphing.
I agree with Brian. Agents are part of the old model and can no longer help authors.
Submit your work directly to editors who can buy your work, then, when a contract comes your way, hire a good IP attorney for a flat rate fee to help you review and negotiate the contract.
As for age. Fifty is young for writers. If you send an editor a story they like, they'll buy it. Age will not be a factor, especially middle age.
As for traditional publishing being done and out -- I'm not there yet. I think there will be a few more years of shaking things out, (same with book stores, and depending on what that model ends up looking like) but I think they will be around and be an important part of the publishing picture for some time to come. Just different.
Fatal Destiny - a Grace deHaviland novel
Yep. I don't think age matters, unless as I.J. says, you're writing a series and you're eighty-two and pressin' . . .