What age do small female go into heat?

VETWEST says:: "Puberty or sexual maturity in the female dog usually occurs around 6 months of age. The smaller breeds tend to go into oestrus or "heat" earlier & some females can have their first "heat" cycle as early as 4 months of age. On the other hand, the large & giant breeds can be up to 2 years old before they come into heat for the 1st time." REF: https://www.vetwest.com.au/pet-library/reproduction-in