Final answer:
To start the sentence with an adverb, use 'At five years old,' making the revised sentence 'At five years old, Vince's sister was born.'
Step-by-step explanation:
To rewrite the sentence 'Vince was five years old when his sister was born' with an adverb at the beginning, you might use an adverb of time such as 'At five years old,' which modifies the main verb 'was.'
The rewritten sentence could be: 'At five years old, Vince's sister was born.' Here, 'At five years old' functions adverbially, setting the time frame for the sister's birth relative to Vince's age.