If dating includes sex (or any form of sexual activity) then the answer is NO - and parental approval is irrelevant. If you are under 16 then you may not lawfully engage in any sexual activity (not just intercourse) with anyone.

Is it OK for a 15-year-old to date a 12 year old?

No, it is not illegal due to just your age difference. It only becomes illegal when there is sex involved, the person you are having sex with is under 16, and you would be either 1 )four years older but less than eight years older than the complainant

What age should a 15-year-old date?

Consider their emotional maturity and sense of responsibility. For many kids, 16 seems to be an appropriate age, but it may be entirely suitable for a mature 15-year-old to go on a date, or to make your immature 16-year-old wait a year or two. You can also consider what other parents are doing.