For the most part, the ceramic part of the toilet, the tank and the bowl will last for generations and as long as the ceramic does not crack.
The ceramic of the tank and bowl rarely crack due to old age, but it does happen sometimes.
Some things that will cause the tank or bowl to crack are, leaning back to hard against the tank when sitting on the bowl.
Too much hot water getting into the toilet from a defective mixing valve or a bad flapper in the tank, that causes the water to get to hot in the tank.
If the bowl is not secure to the floor, the rocking can cause the ceramic to crack.
Using any drain cleaning chemicals in the toilet will cause it to crack.
I have broken a toilet using a toilet auger, it happens.
Sometimes a crack is not visible and is inside the bowl it's self, this can be hard to figure out because the symptoms, are the same for a crack in the bowl and other things.
But if there is a crack in the bowl, the water level of the bowl will drop, sometimes all the way down, sometimes part of the way down, but it will always go down to the same point.
Sometimes it may take an hour for it to go down but, it will always go down.
Two other things will also cause the water in the bowl to go down.
1 is a clogged vent or drain.
2 Is a wicking affect, if something like a paper towel or other, is hanging out of the toilet outlet, it will slowly draw the water out of the bowl.
So that covers the only reason you would need to replace a toilet, because it got old.
As I said before all the toilet parts can be replaced, from the base to the flush handle.
Most toilets can be rebuilt and work just like new.
Some things may cause it to flush bad, like Lyme buildup in the toilet trap, but that can be cleaned.
If something gets in the inside of the toilet water section, if may prevent the water from spinning the water in the bowl, the obstruction needs to be removed.
Other reasons to replace an old toilet are.
They use 3.5 gallons of water per flush, new toilets use 1.6 gallons or less each flush.
The tank top got dropped and broke, and you cannot find a replacement.
You can't live with the color anymore.
I hope this was helpful.
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