I'm 14 with a 6 inch penis. Will I be 8 inches at age 18-20?
That would be impossible to predict. Age only roughly tells a person where you are in development. That is why doctors talk more about stages of development than age. The penis grows in length mostly during stage 3 of development, the same stage when you shoot up in height. How long it grows depends on your genes. While a number of theories have been proposed, so far no one has come up with a way to guess how long a man's penis will be. Once you reach stage 4 your penis may continue to get a bit longer, but mostly it will get wider. When you reach stage 5, the growth will stop.
The average male penis is 6.5 inches when erect, so you will definitely be in the normal range. Having a larger than normal penis will be a bit more difficult for your wife when you get married, so extra care will be needed at that time in learning how to have sex.