We've got 2 cats one is 4 years old & the other 3 years old. We had the litter tray & food tray in the bathroom. We've now got a cat flap & r putting the trays etc outside on the balcony.
I've done this with numerous cats and never had a problem. Just hold the flap up for them at first and gently push them through. They soon get the hang of it.
I have NEVER tried this, though! How to Train Your Cat to Use the Toilet - Info Barrel