I had a 2012 Tonykart that I raced for half of this year. It was definitely wore out. We switched to a new 2014 in the middle of the year and were instantly much faster. The old one was also unresponsive to small changes and constantly had understeer at the exit of the corner.
The previous owner had raced quite a bit on it, mostly TAG. I would imagine that had more to do with it than the year of the kart.
On the new kart, we had to remount the exhaust as the middle of the frame was actually “drooping” even without the driver in it on the old kart. The other visible thing you could notice was if both karts were on the ground and you tapped the front bumper with your foot the old one would bounce off the ground and the new one would barely move. The seat brackets would also constantly break and we’d have to re-weld them back on.
I would also imagine every kart is different as to how long it stays competitive.