Right now I'm 16 years old and the lowest note I can reach is a very forced B1 (not vocal fry). I have been able to reach down to an A1, but I can almost never reach lower than A#1 at the moment. What can I do to extend my voice to a G1 (not vocal fry), just around 2 notes/5 seminotes lower? Wait to get older? Or are there techniques to master?
I would personally recommend Brett Manning's Singing Success program. It helped me increase my range a full octave. I think you would find the same success if you stick with it and practice daily.
The only downside is that is pricey and you have to be willing to pay. Although if you can't afford it, Brett Manning studios and Singing Success has some great tips and singing techniques on their youtube channel for free. Good luck and keep singing!
Bass voices continue to mature and deepen into their thirties and beyond, so your true potential as a bass singer depends on maturation and on training. A singer develops muscle strength and mass as well as muscular skills. As you mature and develop more muscle, you need to learn to use it. This is a process that takes time and work over many years.
As Paul Plishka said, the young performing bass (in his 20's) may add a bit of acoustical darkening to the voice to sound fuller, but as he gets to the late 20's, this added darkness is no longer necessary and has to be taken out of the voice production, or you risk becoming too throaty and dark. Most basses need to think about lightening and brightening the voice. The voice is dark enough on its own and needs the brightness for flexibility and carrying power. You sound good to yourself as you get darker, but others hear it as too woofy and wooly. Careful with a too depressed larynx and a too pressured tone.
Many authorities on singing believe that a singer should be sparing in his use of both the highest and lowest range. Standing around banging out your lowest or highest notes endlessly is going to be counterproductive. These need to be touched and exercised every day, but not abused. Training for endurance and power comes later and is best undertaken with a good teacher, or after you have learned a great deal about how a voice, and your voice in particular, works.
Something that few people know is that exercising the whole voice, and, in particular the head voice/falsetto, is as helpful to basses as to the higher voices. Exercising the head voice and working on bridging the passaggio is critical to developing a classical singing voice and it will also help you sing lower and fuller. While low notes are the special glory of the low bass, he must also sing high notes ("money notes") too. Even if you are a basso profondo, you must still develop a range that at least reaches F above the bass staff to sing the normal operatic and song literature.
Another thing to consider is how the mixing of the registers contributes to a voice with many colors. While the deep, dark tone of the authentic bass voice is nice, it can be monotonous when it's all you've got. Using other colors and soft dynamics is healthy for the voice and also beautiful to the listener. The hardest thing to learn is how to sing a carrying soft tone, but it is also the most magical thing for any singer to display.
While we basses love our low notes, it is difficult to sing the lowest notes loud enough to be heard unamplified in a theater. This is why basses may sing a fifth below low C (that's the C two ledger lines below the bass staff), but the operatic literature has few low notes below low E, and why there are only a few low C's in the whole repertoire. A good contrabass section in a chorus can make these lowest notes heard as a reinforcement to a choir, and the reverberation in the old cathedrals helped this, particularly those with domes. If you don't have this kind of acoustic, the lowest notes will be lost on the hearer. The lowest bass notes are kind of a stunt and can be used solo with a mike, as they may be in country or gospel music.
Finding a good teacher, or at least hanging with singers who know their voices is a great help to the developing singer. Get out and sing for people and find the folks who seem to know the most about the voice. Even a few lessons with a truly great teacher can be a great help. You may find a teacher who is interested in you for your vocal potential. Even a good teacher doesn't run across a lot of potential basso profondos in a career...so you may be able to find someone to help you develop...