[RiseofAsia]

Many like to set to 21 and below is perhaps due to ageing aircon.

Actually, it is lazy to service the aircon.
Once you have regularly service your aircon.
Your old aircon will be cooling even at 25 degree.

One simple way to make air con cooler is to use a brush to brush the compressor fan coil dust away(It will be even better if you spray water on the compressor fan coil, but got to do it during rainy day if you are living in HDB. Need to switch off aircon power first. Do not spray water near the compressor electrical area, just spray on the big compressor fan coil will do.