First off, I'll say that at that age it's illegal pretty much everywhere (I can't think of anywhere it's legal.). Second, all societal norms aside, if parent and child have sex it changes the relationship forever--a few for the better, but most for the worst. Finally, it can have other long-term effects upon all involved.
My own story occurred over 40 years ago. I lost my virginity to my now-deceased mother. At the time I was 14 and she was 40. Dad had basically been gone since I was 4 or 5, so with the exception of an occasional date or boyfriend, it was just mom and I. It didn't help that in the summer, mom would often parade around the house when she got home in just her bra (or a slip) and panties, especially as I entered puberty. Her answer to the "bird & bees" talk was to get a subscription to Playboy after her friend, who was also a single mom, found her son's stash of Playboys, cigarettes and booze. Mom also had a habit of having a few drinks every night "to unwind." This didn't help with her inhibitions. There were other factors that ultimately led to us having sex, but my hormones and her being tipsy if not drunk were the two biggest factors. Over a couple of months things built up between us, then one Saturday afternoon it happened, we had sexual intercourse.
Immediately afterwards we both felt guilty and withdrew from each other for the rest of that day. I don't think either of us ate dinner that night. I know she drank more and got drunk. I couldn't sleep at all. Then, around 3:00 or 4:00 the next morning, she came to my room. She was sobered up and we agreed to never tell anyone and to never do it again. However, within an hour we had finished talking about what happened and hugged each other then we went went to kiss, it was like the switch got turned back on and we had sex for a few more hours.
When we woke up we decided to try and talk about it again over lunch, but it did no good and it started a sexual relationship that lasted until I got engaged well after I graduated from college. Even if one of us had a significant other, we still had sex. She never remarried.
Once I got married it stopped for about 10 years. Then one night we were alone at her house and it started up again. When my wife and I separated, my mom visited often and we always had sex. Then my wife and I got back together, but trips to my mom's house involved sex when it was just the two of us.
Now that my mom is gone, I miss her obviously; but the fact that we had sex really messed things up. I often fantasize or replay sex I had with mom, sometimes even while I'm having sex with my wife or other women. I actively seek other women who want to role play mom-son sex.
I will close by saying if your a teen, it's mostly hormones. If it starts controlling your thoughts and/or you start doing sexual things, you need to stop and seek help. If you're a parent, be a parent, take charge and stop it before it starts. While it may well take care of immediate urges and desires, the long-term effects can be bad.