I don't mean to talk out of turn (given I'm not even a decent chess player, despite playing for 25 years) but I would like to point out that, when saying that it's "statistically impossible", "nearly impossible", etc., you forget that it actually is "statistically impossible" to become a GM at all in the first place, no matter what your age is. Take women GMs for example: What was the chance for a woman to become a GM before the first woman did? Below zero? Statistics are just approximations of sorts, indicating what applies to some abstract "average case" scenario. They definitely don't tell you what really happens -- in that case, the world would already be over.
how old is too old for GM?
What is "the main factor" for making a car function? The engine? The energy supply? The wheels? One can worry about that sort of thing, but it seems to me that age is a reasonable topic. As one gets older and continues to find oneself nowhere near GM-level, it is understandable to think about what might be realistically expected from a major increase in effort.
I love how all these threads keep droning on about this theme. As though age was the main factor in becoming a GM.
What is "the main factor" for making a car function? The engine? The energy supply? The wheels? One can worry about that sort of thing, but it seems to me that age is a reasonable topic. As one gets older and continues to find oneself nowhere near GM-level, it is understandable to think about what might be realistically expected from a major increase in effort.
It may well be a reasonable topic, Spongey, but the point is that it's pretty unimportant compared to the time-and-effort investment involved. Something that an awful lot of people seem to take awful lightly in their discussion of these matters...
Is there a chess.com rule requiring some particular level of importance for a discussion topic? Would you like to cite any particular example of a post where "the time-and-effort investment involved" was taken "awful lightly"? In the opening post, NM darkunorthodox88 indicated an interest in a discussion where "we are assuming here the ... dedication to go about the attempt to proper way". Is it your view that people shouldn't participate in a discussion where that assumption is made? Even with that start to the discussion, weren't there quite a few references to "the time-and-effort investment involved"?
"... dedication/diligence ..." - AntonioEsfandiari
"... study chess 10 hours a day for 10 years... ... forsaking his professional life, social life, family, freetime ..." - maathheus
"... work really hard for improvement ..." - Taskinen
"... truly dedicated and has the time and energy to spare ..." - SmyslovFan
"... dedicated to do hardwork …" - 64-PGY
"... working hard …" - IM pfren
"... time and dedication … large investment of time …" - Dsmith42
How many discussion participants need to mention this in order to satisfy you? What do they have to say in order to satisfy you?

Kmoch's Pawn Power in Chess brought me from 1800 (OTB) to 2000 (OTB).
… or at least, that's the only book I studied seriously as I went from 1800 to 2000.

There are many examples :
GM Aagaard - He was 1700 at the age of 18
GM John Shaw - Same as Aagaard
GM jonathan hawkins - was 1700 when he started university (now he is almost 2600 *2590)
GM Igors Rausis - got his highest ever rating (2651 and top 100 as well) this year (2018) at age 57
IM Eric Kislik - got his first ever rating at age 20 and made it up to IM while having a full time job.
GM Maurice Ashley - barely knew how the pieces moved at age 15
So I think there is no point in buying into this age stuff which, basically , is a myth spread by idoits like "SmyslovFan". These people have no goals in their lives ( rather than jackin' off to spongebob) and spread negativity and discourage other people to do someting.
If you are dedicated to do hardwork and have love for the game there is nothing that's gonna stop you from becoming GM and beyond.
My best wishes
That's pretty cool that you collected this data!

It is easy if you if you believe it is possible! It is not possible if you believe it is impossible!

Perhaps I can offer my youtube chess channel? I analyze master games and show how I analyze my own games. I also have giveaways with nice chess material and finally I will play subscribers! https://www.youtube.com/channel/UCAPbr61Lrt2K3m97w9Zj5vg/featured?sub_confirmation=1 It should help on the road to GM! check it out, you will not regret it! Kind regards, Noam

I, on the other hand, will be world champ (at least) in about 2 and a half years from now from only practising in my PARENTS basement.
Okay jokes aside, as to your main question, I'm not the best chess player by far (I'm stills beginner, and a poor beginner at that) but here's my two sense-
If there 1800 at 16 I would think they could make it. I guess it depends on the person and weather they're stuck at 1800 or still improving.
For a 20 year old 2000 I would say they'll struggle greatly to become a GM. They very well might become a master of some sort, but I'd say GM may be a bridge too far- again it depends. I have the same answer for 2200 at 25.
I think your over estimating the strength of a 1800, im 13 and around 1700 USCF but I really don't think I have a chance at GM, maybe other titles of course though.
GM's that STARTED chess at your age exist you know. It's statistically unlikely for virtually anyone but you are exaggerating the difficulty a bit with that attitude.
Seeing as the person you're replying to is 2500 in blitz 3 years later, I think we can confidently drop even your first sentence.
See
https://en.wikipedia.org/wiki/Larry_Kaufman
He became GM by winning the 65+ world championship

It's a good question for coaches who have worked at that level.
Not a great question for armchair analysts.
If you just want my intuition then... if a person doesn't already have a title at age 18, then I'd think it's too late... and since people with a lot of potential (who work hard) improve a few 100 points per year, we can just work backwards from that. Age 17 around 2000, age 16 around 1800, etc.
And so most important is that rate of improvement... let's say a 16 year old is rated 1500 OTB... but improves to 2000 OTB in one year. Obviously they have a lot of potential.

People are naming the exceptions that prove the rule. It's generally acknowledged that players peak in their late 30s. Will lower rated players keep playing until then? Unlikely, since it's so hard to make a living playing the game.

I don't mean to talk out of turn (given I'm not even a decent chess player, despite playing for 25 years) but I would like to point out that, when saying that it's "statistically impossible", "nearly impossible", etc., you forget that it actually is "statistically impossible" to become a GM at all in the first place, no matter what your age is. Take women GMs for example: What was the chance for a woman to become a GM before the first woman did? Below zero? Statistics are just approximations of sorts, indicating what applies to some abstract "average case" scenario. They definitely don't tell you what really happens -- in that case, the world would already be over.
You're conflating the probability of a specific person, chosen randomly, becoming a GM and that of a population producing GMs. Statistics can be easily manipulated to lie.
There isn't a limit.
yeah there is. if you start playing chess at 13 you wont become a GM honestly
Now your just hating on yourself I actually started way later than you I have a bigger chance at becoming a gm than probably most people here ( so do you )
@Pfren doesn't seem to know how to talk pleasantly online.
I agree with him that studying Bronstein's Zurich 1953 is an excellent way to improve. There are many excellent game collections out there, and if you spend several hours on each game in the book, you will definitely find it worth your while. The players, top GMs all, spent several hours playing the games. The annotations are first rate, and worth analyzing.
The more time and energy you devote to game collections such as Zurich 1953, or Alekhine's annotated best games, or Capablanca's, or Tal's, the more you will improve. If you just read the books casually, you will gain some benefit. But @Pfren is right: these books deserve more than a cursory reading.