Water heater maintenance

I have a 15 yr old Rheem that had the dip tube replaced in 2000. It's been years since I drained it and nothing else else has been done for maintenance. Is there anything I should do now to prolong the life or is it too late and just let it run? (I've read some stories here about people trying to drain or replace the safety valve and the things just breaking on them.) Thanks for any advice.

My rule of maintenance for water heaters is;
1. Install the heater.
2. Replace the heater when it starts leaking.

Draining the heater monthly may let a heater last 120 months or more, but if you don't do it, it may only last 10 years.

If you are willing to replace the anode rod periodically, the tank might last longer. Mineral deposit buildup slows the recovery and lowers the volume over time, but doesn't really shorten the life. If you actually check the T&P valve and it doesn't reseal, then replace it. If it has never been activated, doing so manually will probably cause it to leak. If you do it regularly, it might not, but still could start to leak. It gets some mineral buildups on it, and can't close itself properly. If it gets really stuck, it could be a safety issue, as it might not be able to open (that's why you should check them periodically) when it should, if something goes wrong.

There was a period of time that many dip tubes failed and yours was probably one of them as was mine. It seems dip tubes were, at least at one time, manufactured by one company that supply a number of manufacturers, and for some reason their tubes failed in large numbers. I replaced mine, and the heater lasted for a number of years after that. I think at this point, you should just check the TP valve, replace if it doesn't shut off after testing, and go on until the tank starts of leak.

At 15 years there is nothing that I would touch on that water heater.
Test the T&P is the extent of what I would do...