According to Shariah, a specific minimum age has been determined for an animal’s sacrifice to be deemed valid, and this requirement differs from animal to animal:
1) Goats, sheep, and rams should be at least 1 years old
2) Cows, buffalos, and bulls should be at least 2 years old
3) Camels should be at least 5 years old
An additional detail is that if a sheep is 6 months old but appears as healthy and well-fed as a 1-year-old, then its qurbani will also be considered valid.
If a person has witnessed an animal growing and can confirm that it has reached the minimum required age, then its qurbani is considered valid, regardless of whether its permanent teeth have fully emerged or not. This is because the condition is for the animal to have reached the minimum age, rather than for its permanent teeth to have developed.
However, in today's corrupt era, one cannot rely solely on the word of a seller. Therefore, as a precaution, the growth of permanent teeth has been established as the standard for determining whether an animal has reached the minimum age requirement or not. Almost all sacrificial animals grow two permanent teeth when they reach the minimum required age. It is not possible for an animal to grow two permanent teeth before reaching the minimum required age, but it is possible for an animal to have crossed the minimum required age without yet having grown two permanent teeth. Therefore, this method of checking the teeth effectively confirms whether an animal has reached the required age or not.
In summary, if the age of an animal is not confirmed, the growth of two permanent teeth should be used as the standard for determining whether or not the animal has reached the minimum required age.