Many Aluminium presure vessels suffer stress fatigue over time. The life expectancy seems to be 10-15 years. I can vouch for 10 years and aluminium being a good idea: I had a well-used 10-year-old aluminium-framed mountain bike that had a frame failure (main tube snapped) due to the fatigue of years of use.
Weihrauch recommend disposing of the HW100 air reservoir after ten years for safety reasons.

I'd say if it's ten years or more, get a new reservoir.
I'm sure there will be people come along and say that they're been using one for nearly 20years. Good luck to them. I wouldn't take the risk.