Well actually it was written for The Wonder Years, so they wanted it to happen. Maybe it wasn't so spontaneous. But again, what 14 year old boy can say he kissed a 20 year old girl. Even if it was his job? I know, this sounds weird.
But that's not the point here. I was talking about Holly Fields here who got the part of Jennifer at the age of 12. She does seem a lot older than that. Maybe she was quite mature for her age. I don't know.