Hmmm house was built in 1999, I bought it in 2015, and the crack only started developing about 2 months ago. Not sure if this is the original floor put in in 1999 or if the previous owner redid it.Depends on the reason it is cracking. If it is cracking because the floor is "moving" any fix will be temporary unless the fix addresses the root cause. Floor moves, non-flexible things crack.
Stopping (or mitigating) the floor from moving (I mostly mean up and down) is likely no trivial task and may even be due to how or on what the tiles were put on. Sub-floor not properly attached to the joists, joists too small, old structure, no decoupling membrane, etc.
The best practice/methods for what goes under tile has also changed over the years....
If it is the case depending on how much movement there is repairing the grout will be temporary but may still be worth while (depends on how long it will last). Fixing the root cause 100% may be a huge job....
If I have to repair a crack every 5 years that's no biggie.
I wonder if there's such a thing as a flexible grout that would work better in such a situation.