Is sentence No3 a fragment that needs to be attatched to No2? [duplicate]

I thought I heard my teacher said: 'Don't construct a sentence without a verb.'

Is there such rule in english grammar?

She looks like 70 years old. Perhaps older.

How about the question above? Is it correct? Is there any book that can be used references for wide range of topics in grammar(including syntax, morphology et cetera)? I have finished oxford grammar in use recently and think it wasn't enough for me.

The usual definition of the word 'sentence' demands that there be a verb*, so the question becomes nonsensical.

However, rephrasing to 'Don't use structures other than sentences,' we must say that this is a very shaky rule-of-thumb rather than a grammatical requirement. In conversation, sentence fragments / sentence substitutes such as Tomorrow, Yes, If you must, On the table are often given as answers and are quite acceptable. Follow-on fragments are also quite acceptable in informal writing: She is 70 years old. Perhaps older.

Look up articles on 'sentence fragments' and 'crots' on the internet for guidance on actually using these structures.

*The fact that 'sentence fragments' are sometimes called 'minor sentences' is, I feel, confusing.