Everyone is different, but don't rush anything for vanity because you can really cause some damage to your body. Giving birth isn't just any little thing. Your body went through a lot and your doctor can better tell you when it's safe for you to exercise. Wait until your post-partum check-up
It should work itself out. I was 19 when I had my first and lost the belly pretty quickly.
Your 16? Oh dont worry, you are young and your tummy will bounce back in about 6 weeks or so. Dont get discouraged it will happen. My mom was 16 when she had my brother and her tummy was flat as a board. It just takes time.
It took nine months to stretch those muscles and it will take a few months for them to go back to normal. don't start exercising until you are cleared by yoru doctor to do so.
It usally takes 6 weeks or longer to get your body back in shape and if you got stretch marks then you will probably never get the flat stomache that you once had but after the 6 weeks is up you can start working out and going back to your normal routine.