What rating can I ultimately hope to achieve?

Not that rating, in and of itself, is super important, but Ive always been a sort of "all or nothing" kind of person and it would bother me to think that my knowledge and understanding of chess somehow has this inevitable cap on it just because I didn't happen to start playing when was 6.

I started playing seriously just a few months ago. Im 28. I currently spend ~32 hrs/wk on chess, whether its reading books, playing live/correspondence games, tactics, etc.

If I work hard and stick with it, am I doomed to plateau in the range of mediocrity? Is excellence impossible?

What is it that typically keeps people from progressing when they hit a certain age? Is it the demands of adult life? Is it that they haven't trained their brain nearly from birth and its just too late? What causes this?

There is no age limit to learn something new.
Many people start a second career at the age of 40 and excel at their new trade, like a late bloomer.
I believe a rating is a bit like a school grade.

±1000: 46 % = E (Developing)
+1100: 50 % = E+ (Bare minimum)
+1200: 55 % = D (Progressing)
+1300: 59 % = D+ (Below average)
+1400: 64 % = C (Average player)
+1500: 68 % = C+ (Satisfactory)
+1600: 73 % = B (Good player)
+1700: 77 % = B+ (Very good player)
+1800: 82 % = A (Superior, above average)
+1900: 86 % = A+ (Excellent player)
+2000: 91 % = (Expert)
+2100: 96 % = (Candidate master)
+2200: 100% = (Master)

Chess is like playing the piano. Some play with one hand, while others with two. Only masters play with all their fingers and with all their knowledge, they are able to harmonise everything together to make it look so easy.

Is there a lot of late bloomers in chess?

Yes, it depends on when you started to play. And also it depends on "did you trained your brain before".
For example, somebody, who paint very well often can become a great writer, because this, of course, arts are connected.
Usually, if you didn't train your reaction, imagination, intelligence until 20 years, it would be very difficult later.
If you are very good in some area, may be your intelligence is very high, because of your work, (if you are great programmer, for example) then development of chess skills will be much more easier.
But still, if your started to play at 28 you could, probably, become a IM, may be even GM (though it will be quite difficult), if you will work very hard and use all your free time (for 5-7 years). But I don't think you could become like Magnus Carlsen or somebody like him.

But I know one man, who didn't actually did sport professionally, but in 54 he made a world record in swimming. Or you may be now the story about Cliff Young, who won marathon in 61 year against young sportsmen. So, who know, may be you will become new world chess champion.

I wouldn't worry about it. I learned all that I know playing chess here, and by briefly going through tutorial on the last chessmaster.

Before that I just briefly knew the game.

Here's something interesting. When i was young (20s, 30's) I spent a lot of time studying and playing tournaments uscf. My rating was around 1500 uscf. Couldn't get any higher. Then when i got too busy, i quit chess for 20 years. Never looked at a book, played a game, nothing. Then when i retired i began playing again. Walked cold into two or three tournaments and my rating shot up to 1700-1800. Either i got smarter or the general population in got dumber. I never did arrive at a satisfactory explanation other than when you get older you get more careful and that's an important aspect to the game. Go figure.

Forgot the openings, I learned the basics from this page:
http://www.chessgames.com/perl/explorer

Toscani
Your elo rating strength is for Lichess rating, or is for general rating?

I think that your approach to chess is incorrect, You dont need
to be a grandmaster to enjoy chess,
chess is about persistence, time and patience so You wont make it in a year and nobody does it,
if You think to try to be like the next Kasparov You will probably be discouraged and leave game.
Chess is a humility game when You face the chessboard You know it is unfathomable, I mean everybody has this,
do you think a grandmaster enjoys playing an engine?
and be beaten and be beaten again.
The aim of chess is not to grasp it all at least for me because It is impossible but rather to experience it, to enjoy it, this is a game with great rewards but You have to work hard to find them,
I started myself as well in the 20s two years ago, so I dont expect to be like a master or something like that,
I have more like a reachable goal like becoming 2000 in 10 years but I dont even really care I dont even compete,
I dont really care about the points.
I care about the position, I want to know what the position tells me, so there is a moment that You enjoy chess because You enjoy chess and of course if You get tired of playing there is nothing wrong of leaving a game Its only a passtime but It can also be a long live hobby.
This is a century game, It is part of the history of humankind, in India, I think It started in India.
many people have spent huge hours and huge brains trying to grasp its secrets, so You are not going to become an all chess player with that attitude, I mean you have to go slow learn first simple tactics, checkmates and It is not easy but that is the joy, if You want an easy game just look somewhere else and remember You dont have to be a Master to enjoy it,

Do you think that It is worthless for a student to learn englihs since He will never talk like Shakespeare, of course not, that is a ridiculous assumption
I mean I dont have anything against your approach but It does not work like that in chess if You want to excell find an easier hobby or be prepared to work 10 years or so.
Or a piano as a post above said, You can learn to play
a piano even if You will never be like Chopin or a famous one, or a soccer player, million of people play
soccer and none of them play like Messi or Christiano Ronaldo.
So you get the point,
I want to be explicit about this because I think It is really hard to try to know how hard the game It is
everybody knows that Christiano Ronaldo is a great soccer player or Michael Jordan or whatever but It is not easy, specially for a newbie to appreciate the huge effort that the top players do to reach the top,
I myself cant do that, I know that they work a lot, that they spend a lot of energy on the board but chess has this quality
that only those who are close to excellence can understand the excellence,
You see snooker for example and You see that someone plays well but I am sure that a newbie can not tell the difference between a 2200 player and a 2800 player and It is exponential, so this is why It is important to say it and say it again, because the board looks small It has only 64 squares, so one might think that It is reachable but then You find that the possibilites are tremendous, the wikipedia article says more than the atoms of the universe which I think It is an exaggeration but anyway You get the point
the game is harder of what It seems.
Check this
http://en.wikipedia.org/wiki/Shannon_number
In summary chess is difficult but It does not matter, Its fun.
Or as the quote says "life is too short for chess"

^"Or as the quote says 'life is too short for chess' "

Unlike your post! Haha..

@Curlaub: There is no known, definitive answer to your question. I personally think that if a person doesn't learn how to play before their brain stops developing, then they are at a significant disadvantage.

I say excellence is NOT impossible, but improbable.

You can certainly get "good", though. Carry on.