Is it fine for a senior to date a sophomore?

No, its totally normal. I was in high school not too long ago and there were many Senior/Sophomore relationships, with the guy being older the vast majority of the time.

How old is a sophomore usually in high school?

The 10th grade is the second year of a students high school period (usually aged 15–16) and is referred to as sophomore year, so in a four year course the stages are freshman, sophomore, junior and senior.

Can an eighteen year old date a fifteen year old?

In a Nutshell: Sex between an 18 year-old and a 15 year-old is illegal in California, meaning any touching of “private parts” of either, even with consent of both parties (and perhaps even the parents of each teenager) is illegal in California because the age of consent is 18.