I would agree with LidoCA - when it gets to the point where it's more of a liability than enjoyment, it's time to let the scooter go. Until that point, it is a bit of a challenge to see how long I can keep it running without putting a ton of money into it. I have an old push mower that should have been replaced about 5 years ago, but it's kind of a challenge to see how many more seasons I can get out of the old girl with minimal repairs and cost.
Trick to keeping a mower alive for decades is to:
1) Never change the oil
2) Never drain the gas - nothing magical happens over the winters
3) When wheels break out of the chassis, drill new holes OR add real big washers and go back in that hole
4) Mowed a whole summer perfectly well with a trike mower
5) The blade will eventually hit enough stuff to begin re-sharpening itself - magic!
6) If you need to use your sneaker to shut it off, something must still be arcing down there - so spark plug is still good
Also, I really baby my mower by using nothing but premium gas!
Stig