Goats are full grown goats by the time they reach age 3 years old.
Any goat that is under 3 years old is considered a kid and any goat under 2 years old is usually considered a baby goat.
Goats grow up pretty fast and so within 3 years you'll have a fully grown adult goat which is no longer a baby goat.
With a human 3 years old would still be considered a baby by some people and the 3 year old human would still need lots of care and attention to watch them.
But a goat at 3 years old is fully grown and can go onto do things independently without the need for supervision or care from the mother goat anymore.