First, I am one of those fire service guys just not in his area. The 12-year rule is for disposable extinguishers (look at the gauge if it says discard/replace its disposable) and not for any that are designed to be recharged or have internal maintenance preformed at six years (gauge will say recharge). The USCG will allow recreational boaters to service their own extinguishers for both non-rechargeable and rechargeable extinguishers commercial boats need a certified person to inspect rechargeable ones. If you choose to inspect your own do yourself a favor and be sure you know what you are doing and not just look at the gauge and say yah it looks good. In regards to the fixed system like Fireboy or Seafire that are found in engine rooms these need to be annually weighed have any shutdown and manual pull tested and, have an external 5-year visual inspection by a DOT certified company.
With that said even though many extinguishers say they are rechargeable many either are not or should not be serviced. An example is the First Alert extinguishers sold at Costco these have a pink colored powder made in China for ABC as opposed to yellow found in every other one out there also there are no factory parts to service them. I’m not sure what color pink and yellow make but it will not mix well in the recovery unit. If you find a service center willing to service these I would run not walk away. The Kidde brand that are very popular because they are cheap have had many units recalled into the millions. If you have a Kidde brand, I would encourage you to do a search for “Kidde Extinguisher Recall” and be sure yours is not on the list.
The thing I hope anyone reading this takes away is your vessel fire protection is not a place you want to be frugal. If you have a fire while away from the dock remember you are your own fire department, and you may want a professional to have inspected your equipment and buy quality. Ask your insurance company they may have a say in who can inspect them also. Instead of the box store or little cheep one sold at most marine stores go to a fire protection company and buy a quality one or more. I recommend to get more than the USCG minimum, not just because I sell them but if you need it more is better than less. If you have invested a lot of money in your electronic package I would recommend a clean agent like Halotron as it will not damage your electronics like ABC powder.