The correct way to rewrite this sentence would be by adding the word "then", which is an adverb of time and a connector that expresses the continuity of time, and also by making the necessary changes. The sentence describes three stages of an elephant's life and how its tusks are in all three stages. So, the sentence asks for a connector that can cause the continuity effect, that can express a consequence in time, meaning, "after that". Also, it is necessary to add the personal pronoun "They", referring to the subject "elephants", in order to make the verb "to get" in the sentence become grammarly correct. Moreover, the subject "Elephants" should be written without the apostrophe, because the "s" at the end of the word configures plural, not possessive. The corrected sentence would be:
Elephants are born without tusks, then they get baby tusks and replace those with adult tusks.
