If I started playing Chess today in the age of 21. How much time would it take to play like a GM?

Like GM? I do not know , but I would say one thing : why not try ? Why not try to work hard every day and do your best to achieve let’s say NM or IM . I’ve seen DOZENS coaches (NM, IM ) who started chess at late stage of their life like 18-21. So why not, instead of asking people’s opinions , go try things that seem not possible but CAN be reached within hard work and A LOT of fail.  

It's viable. It's more likely you'll sizzle out at FM but FM towards IM is definitely viable. GMs need to outplay other GMs as well as consistently defeat IMs to gain the Norms sufficient to gain their title. That doesn't just take skill (which it takes a massive amount of) but more importantly involves you no longer being a name on a screen. Fame has drawbacks; severe rivalries and pressure aren't for everyone. Make sure you understand what you're entering. It's definitely doable, you'll just be a rarity is all. Aim for FM, if you achieve that then aim for IM. IM is an extremely respectable title for adult-bloomers. Don't feel remotely bad or underachieving if you end as an IM rather than GM.

The truth is simply that everyone plays like a GM because they used to be amateurs too. There is a school of thought that says it takes 10,000 hours of practice to master any skill. One of the articles on Chess.com says GMs practice 20hrs per week on average so that divides down into 500 weeks or about 10 years for the average person. If you're prepared to put the work in then you have 100% chance. If you're not then you have no chance. Most people are somewhere on the spectrum in between.

Hmm

They weren't a GM when they were amateurs so you can't phrase it like that.

Not exactly impossible, I would say if you want to go for it just go for it, just do like 6 hours twice a day with no missed days and analyze all games, develop patterns and your own carved mark in the game, then from there just play people your level and above until you reach what you think was a good accomplishment, if you want to go higher then continue inching your way up! You prolly will get to a Basic GM, but for a Super GM that one I won't make guarantees on, heck you can even try playing 1 human twice and 1 computer twice at your level or above and flip flop till you consistently can draw or win against GM's or Stockfish etc. More than half your games... By studying strong moves you should in theory just be working up the ladder as it stands.

Also change your pfp to a cringey cat girl and make troll posts.

35 years

It's too late starting age, if your goal is to play professionally (to earn a living from simply playing tournament chess, so price money, sponsorship etc.). However you could still become a really good player (even master level, titled player) with a lot of hard work for a few years to a decade. Perhaps you could then make a living as a chess coach or an author, even though it's probably not all that lucrative profession (at least for the majority of people) to make a comfortable living. If you have the personality for it, you could make chess content on social media platforms, where skill level is less important and the quality of content matters more (are you funny, can you edit videos, can you interact with audience?)

In any case trying to make a living out of chess in any case is not very likely. I think creating chess related content on social media platforms and/or teaching other players might be the most reasonable source of income a chess player could expect from his passion. Then again, there are probably a million easier ways to make a living. And there are a lot of other professions where you could still become a top tier professional in your field if you spent that time studying, getting a degree and looking for good career opportunities. 21 is still young in most areas of life, but for a serious chess player it is unfortunately quite an old age to start learning.

Just let him be, for all we know he could be rich and wealthy already, and needs something to learn to kill time, example Elon Musk if he wanted to learn chess for example could drop everything and spend all his hours in study and play, and still be making more interest than all of us on welfare would ever dream of making, even with a part time job in the mix. 21 from a wealthy family prolly means he set for life, and not like us who always end up homeless and broke all the time. So I would say go for it what's there to lose if you already have a steady income source. 

This question comes up over and over and over. Here's my two cents: You say you've just started playing at the age of 21, and you want to know how long it will take to get to GM level. Well, forget about this question for the next two years. Then, at the age of 23, having studied and played for all that time, come back and ask it again.

Here's a sample: Q1--I'm 23 and started playing two years ago. Since then, I have gone from absolute beginner to 1900 over the board in classical time controls. Can I make GM?
A2--Keep going! You're doing amazingly well, and there is a slight chance that you might make Master or IM. GM, though, probably not.
Q2--I'm 23 and started playing two years ago, and my rating on chess.com is 1350 for rapid games, and I've got a score of 25 in puzzle rush. When might I get to GM?
A2--Never. Keep playing. Have fun. A title will never happen.

Stop reading comments, screw the logic and go find a wizard.

Try this.

Awe come on! Seriously!!?

And I thought I was special when I was 10 or so and my parents bought me one of those chess Chess/Checkers games.  I tossed the Checkers. 

I never even got to tour Toronto until I was in my late teens! Damn.

I did go to school with Bryon Nickleoff.  We were in the same grade and same class. Does that count for anything?

Start Streaming on Twitch then? 

Cute wont give you a GM but you can get 1 million views. 

play vs. me I am you are