Many of your questions are answered in detail from many members spotted throughout this forum, if you have time to search through the forum you will find some good ideas.
I am 19 years old is it too late to be pianist?
Do you mean that you want to play/teach piano for a living or just play for yourself and loved ones? Either way the answer is to your question is no, but to take it professionally requires that you understand what niche of the musical industry you fit into and there are certainly many there and those still waiting to be filled.
A music school I worked for was quite large with over 1000 music students every week attending, the man who owned it fixed guitars, but he makes most of his money now from teaching even though he doesn't do the teaching. He started off with a small music store selling/repairing guitars and it grew to a large music school teaching all sorts of instruments with 20+ teachers. So you may want to be a pianist but where it actually takes you you need to solve that for yourself and you may go unexpected directions.
Is sight reading necessary to be a good pianist?
A master musician needs to master many different aspects of music and sight reading for their instrument certainly is one of them. Sighting music for piano in my opinion is probably the hardest of all instruments to learn (next to multi tiered and pedal organs). The better you read the faster you learn, I do not know any master
CLASSICAL pianists who are not excellent readers, if you say Jazz then I will show you a large amount since their music is rhythmic/improvised not so much relying written notes.
The faster you read the more music you will memorize at the end of your life, this is by far a skill that no professional classical pianist would want to miss out on by choice.
How long will it take to be good at sight reading?
How long is a thread of string? Sight reading continually improves sort of like reading words. If you read a lot of words then your reading speed will be much faster than others who do not read as much. If you learn special techniques of speed reading you may read much faster than anyone else who doesn't know such tricks. The same applies for music, if you do not know the tricks then you will never improve your speed/accuracy to a masters rate. Your reading speed/accuracy will improve constantly based on your perception of what it means to read music, you need good direction and advise and a high IQ in pattern recognition in my opinion to read at a masters rate.
The biggest problem with reading is the fingering, if you always use the correct fingering then reading becomes a breeze, however if you read well but your playing is riddled with incorrect fingering you will hit massive problems improving your reading. You also need to be able to play without looking at your hands to be a good sight reader and this takes many years experience playing the piano before the contours of the keyboard itself becomes ingrained into you consciousness. Also the feeling of chords, scales, arpeggios, keysignatures etc etc all cause a physical reaction to the contour of the keyboard for all good sight readers simply when they think about it.
Is sight reading gifted or can it be learned over time?
Anyone can learn to read competently and learn music at their playing level at an efficient rate. I have only met one person who read at an extraordinary rate, my highschool piano tutor who literally read bars ahead of her playing all the time. I remember the first time thinking I was reading wrong when she told me to turn pages and she was still playing a line before the page turned! I didn't believe that she actually could do it and printed off some obscure music and she did the same thing. I cannot understand how to read ahead of my playing several bars (for music that is simple it is possible but not normal standard music) but some people can do it, I seriously believe that is a gift since I still can't crack it with music I have never read before!
Is Any tips to share for sight reading?
You need to work out what is easy, average and difficult for your to sight read now. Focus your attention on reading lots of easy music and pushing up the difficulty until you start making fingering errors. Reading fingering constantly is a bad habit as it distracts you from reading ahead, so you need to ensure that you have good sense of fingering with the music you are predominantly sight reading, but don't make it too basic for yourself, you do need some little parts which challenge you fingering but does not leave you lost. Learn to play without looking at your hands.
You also need to learn lots of music and build your repertoire as this will indirectly improve your sight reading also. The more music you memorize and the more you learn about the routine of playing the piano you will also see the same pattern in your reading and thus things can be absorbed faster.
And how do I know if my piano teacher is good at teaching?
https://www.pianostreet.com/smf/index.php?topic=30543.msg353574#msg353574 
Topic: Questions about sight reading and more.