As long as both parties are 18 years of age or older, not being forced, not unconscious, not too drunk to consent, not mentally impaired to the point they cannot consent yes, an 18 year old and a 21 year old can have sex.

Is it illegal to date someone 21 when your 18?

California – The age of consent in California is 18. It is illegal for anyone to engage in sexual intercourse with a minor (someone under the age of 18), unless they are that persons spouse. California employs a tiered system where the greater the difference in age, the greater the penalty.