わたしは30さいです。/ I am 30 years old.
30さい
Saying someone's age, there is a counter for number of age. It's "さい". You need to put this counter after the number. "30さい" means "30 years old".
わたしは 30さい です。
This is "AはB / A is B" grammer. It means "I am 30 years old.".