Steel cylinders could be used forever if tested & they keep passing, there isn't a final date.
(Unless the cylinder is marked otherwise)
You should see the born date stamped on the cylinder, and the due test date marked on the label.
Expect a 5 year cycle, but you should confirm the cylinder type etc, there could be older ones still about with a 3 year cycle (depending on type they change to 5 year cycles on retest or stay on 3)
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
If you were able to handover for a test and not go to the extra expense of postage then it could be financialy viable to retest a steel cylinder. But at about £40 its just as easy to buy new
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If you attempt to remove the regulator from a non-empty cylinder it will be harder with the pressure, but not impossible.
The regulator thread will have a safety feature that means should you manage to unscrew that it will release pressure (thus you get a spray of high pressure air rather than a flying regulator & cylinder)
Depending on how the regulator is fitted there may be threadlock in there - again makiing it harder to unscrew, and probably needing heat.
Take care not to damage threads, if you are just removing as part of the disposal process then this is less relevant, but if removing to reuse the regulator then it is essential that the threads are not damaged.