Plenty of inexperienced riders come to SE Asia and rent motorbikes. Tons and tons. It is very common and most people are fine.
But motorbike accidents are also the #1 cause - by far - of foreigner fatalities and injuries in most of the region.
It's really up to you to decide the amount of risk you want to take. Being 18, you have crap judgment even if you yourself think you have good judgement the way that most 18 year olds do.
The last time I was in Thailand, I met this guy who rented a scooter having never ridden and didn't even know where the throttle was. By the end of the day, he was zooming by small crowded streets faster than I (having many years of experience) was comfortable with, all the while he thought he was riding carefully. What happened to him? Nothing. He spent a week riding around and then flew back home. But things were going to catch up to him for sure.
The bottom line is that I doubt that you will keep it slow and steady the whole ride and criss-cross the country at 15mph. And the big problem is that you don't know what you don't know
. It would be a lot better if you had an experienced rider with you to guide you and tell you if you are doing something wrong. As it stands, you might be screwing up in some way and not even know it until it catches up to you in a bad way.... or maybe it won't catch up to you... your call.