Can a 16 year old be with a 29 year old?

This means that its against the law for someone to have sex with someone under the age of 16. It wouldnt be illegal for someone whos 16 to have a relationship with someone who is 30 - unless that person is their teacher or in a position of authority. So it would be up to that person to decide whats right for them.

How old do you have to be to date a 29 year old?

Yes--given that the age of majority, in California, is 18-year-old, an 18-year-old female can (legally) date a 29-year-old male.