My hot water heater is going to die isn;t it?

Sounds like (as a minimum) it needs flushed.

-Unless it has been drained and cleaned periodically (the hose bib at the bottom of the tank) then it is continually filling up with fine sludge from the water supply and the decomposition of the "sacrificial anode".
-This causes the VOLUME of the tank to decrease as it fills with foreign matter. Hence your 40 gallon water heater may only be heating 20 gallons. The other 20 gallons has been displaced by the accumulated sludge.

I don't work on gas ones so I cannot advise you on how to safely service them …

But for Electric:
You can shut it off at the circuit/breaker box, get a hose hooked up to the bib and drain it to the OUTSIDE (if it isn't too clogged up). Then pulse the input water (Using the input valve on the line to the tank if it has one OR the water supply shutoff valve where the water line enters the house). A bunch of GUNK should start coming out the end of the attached hose.

Then (if you know what you are doing), you can inspect and replace the sacrificial anode if necessary.

If it says 1998 on the back...it was probably installed in your dwelling within 2 years of that date... so it is probably close to the end of its life cycle.

Where I live .. the water heaters usually last about 20 years if it has been "ignored" over the years. Our water has a lot of "Mineral content"

Good luck.