There is so much piano music available from 1600 to present day.
This listing gives a guide, including the exam year usage, for the standard of a piece.
The standard is a guide. If you are grade 2 you should be able to play a grade 2 piece with practice, a grade 1 piece with ease. Equally so a grade 1 piece can be used by a grade 5 learner as sight reading practice.