There ain’t but about 4 basic parts to a water heater - the control section, the tank and two heater rods. Most have one of the rods go out and have it replaced.
Hard water can do some damage also.
But hey man, after 25 years, maybe it’s time to retire the beast and get a new energy efficient one.
Our’s was about 21 yrs old when we switched it out.
Lucky you! Around here (SE Pa.) we average a water heater every 3 years.