Technically there is no life span for aluminum or steel cylinders. Some shops put a life span on them such as 20 years. But that is a local thing not government or industry mandated.
One could say typical dive shop nitwits but they are concerned about safety /liability. Not sure which ones first.
It's not a bad idea to remove the old Luxfer cylinders from service that are subject to the sustained load cracking problem but there are many of them that are holding up just fine, so that is controversial.
On a 17 year old cylinder, if it passes hydro , what's the problem. It surely isn't made of the old Luxfer alloy. That in my opinion is a shop looking to make sales. mark up on cylinders is pretty pathetic so I can't see any great advantage in doing that.
If you have another shop in the area, try them.
You could take the tank to a hydro facility yourself but if you have to go back to the same dive shop for air they probably won't fill it anyway.
I take my own cylinders to the hydro facility and have my own compressor so I don't have to put up with that stuff. As long as I do the annual visual and maintain the hydro I've done everything I can until I learn that there is a problem with a certain cylinder.
The cylinder is about the cheapest piece of gear we use in scuba. If the LDS has you over a barrel you might just have to bite the bullet. At least take the cylinder to the scrap yard and get some cash for it. I wouldn't leave it with the LDS though and get money for them as they might be using it themselves. Or drill a hole in the bottom.
When I get junk cylinders (even when I give them the $15 scrap value)I turn them into lamps and cut rings from the straight section of the cylinder and weld them into art and sell them. I end up with more money than the cylinder costs so it's a good opportunity for me.