Untill what age can you make it big in chess?

That is a rare exception. For a random player it is actually tens of times more likely not to happen, than to happen.

Agreed.

It's not impossible, just improbable. Agreed that it is MOST improbable, though.

 I'm turning 26 this year, never played chess on an intensive level (I did learn to play as a kid but never played officially) and not planning to make a living out of it or necessarily getting any title... What I can't decide is whether it is worth investing time (and money for premium perks) to increase my skills, knowing that I'll never become really good unless I quit my job and focus on chess only, which will not happen? I enjoy playing but I'm not sure I'm up for the challenge of memorizing a ton of opening theory among other things. And playing without theory memorized won't get me far from the level where I am now, that I know for sure.

I know I won't be a master, that's what I was trying to say originally, but unless there is a chance to become significantly better than I am now maybe all the time required to get there would be better spent on some other hobby which is rewarding even if I'm "too old" for chess

My experience pretty much mirrors what OP said. Reached 2200 strength by my late teens/early 20's and quickly plateaued. Haven't done any kind of serious study for over a decade now, I know for sure that even if I worked 5-6 hours a day becoming an IM (let alone a GM) would be completely impossible.

I disagree. I agree that someone could be a gm at any age, but I dont think anyone could. That's like saying anyone could be a nuclear physicist or NFL quarterback. I think there are requirements to becoming a gm that not everyone has. There are what, a couple thousand grandmasters on the planet? So it's obviously a rare feat. Most people, by far, are either too dumb, too lazy, or too busy to ever even come close. And even most people that are smart, motivated, and have the time STILL do not make it because they simply are not gifted at chess.

nobody knows that. 2Q.

lots of people commit tons of time-- but very, very few become GM.

as lots of people emphasize, its Improbable at any age.  it seems..... arrogant to assume that zillions of teens (many of whom have a great experience in scholastic chess) fall short because of the time they spend studying/training.

now In response... I expect people to rant against video games, girls, and university life (finals and parties)....   but its still merely a rant.  You don't know how many people worldwise are putting in significant time.

You simply don't Know.  this goes to the OP, too.. 

for myself I prefer to think that Improvement is possible at any ages-- and results may vary person to person and depending on the passion and wisdom of their chess study program...   but stuff like "anybody can be a GM " I think is crap!  its a bad message, and it demotivates those that may be improving, but not as quickly as they like.

see I'm not the only one.  and you can't prove otherwise.  lots of people fail, so it could impossible (for them).

[edit] apologies to the OP.  she never did say that it was possible or probable.  in fact she says that improvement past 2k  for a 25+ doesn't happen.   that goes a little too  the other way... but is tons more reasonable than

"anybody can be a gm"

It is wrong. For the vast majority of people once they hit some level they are going to remain at it even after significant time invested in chess. Also, I suspect kids invest relatively small amounts of time in chess, but they make the most progress. 1000 hours invested in chess under the age of 18 and after 30 are going to have pretty different effect on your playing strength.  

In order to become a GM: Maybe start between 3 and 7. By the age of 12 you should already be a pretty strong player...

Questionable. The main problem is not time. The main problem is the plasticity of the brain. I know adults without a job or a family who have devoted the past 20 years or so to chess - without ever becoming masters. On the other hand, all those strong kids who become masters by the age of 14, have hardly spent more than 8 years on chess...

12 is pretty late. I guess Yasser is very talented and an exception.

But can you tell me just one GM who has started playing chess after the age of 21? Just one?

He was already an IM by the age of 21. Obviously he started earlier... It is one thing to be already a strong player and then manage to get the GM-norms later in life, another thing to start learning about the schoolar's mate when you are an adult. 

What Jenium says is solid. Usually an adult 2k+ player can study his ass off, he will remain at the same rating more or less.  

I have this tremendous sense of deja vu.