Jaetee, in the UK system that applies to most of the international schools in HK, primary year 1 (P1) starts at around age 5, but many international schools have nursery and reception years prior to that. These are also equivalent to K1 and K2 in kindergartens, and are for 3 and 4 year olds. For schools which do have reception class, it's difficult to skip that class and do kindergarten instead until P1, because their P1 class will be full with the kids who did reception class.
It seems most people do K1 and K2 here, which they would not in western countries, which I think is partly influenced by the local school system, where the "good" schools do not accept children into P1 who have not attained a certain standard of Chinese writing ability taught in K2.