Most chess players never become Grandmasters, even if they start as children.
If I started playing Chess today in the age of 21. How much time would it take to play like a GM?
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.
I do not believe that at this age Elon musk or even bill Gates could make it to even IM. OP asked a real questions and is getting real answers lol
Elo’s research suggested that most Grandmasters were playing at the Master level by age 18. That was almost 60 years ago. Today, players start at a younger age than back then, and there are more tournaments and both more and better training opportunities.
Today, future Grandmasters are achieving the Master title before age 15. The top players receive titles by age 12.
That being said, I know of some players who learned the game in their 20s or 30s and went on to become Experts (>2000). So, someone who learns the game at 21 could reach the Master level, if they have talent, motivation, and effective training.
I think starting out at 21 and dreaming about becoming a G.M is going to be tough and financially a hard career move, you have to start young and have to abillity very young to make it, unless your hitting 2700 by the age of 21 you won't even get invited to any G.M tournaments and there are so many F.Ms and I.Ms who are logging there guts out at weekend chess congresses for a winners prize of £250. With jumpers with holes in its quite sad, my advice is studdy to be an accountant or a lawer you will earn more money and be less stressed than dreaming of a G.M title.
That's generous, but possible only with living chess 12 hours a day every day and with talent to match, we had a talented youg lad 14, he won tge british under 16s and 18s git his over the board rating up to 2267 went to Uni and got a degree and now works as an accountant firm esrning more than he was as chess player, unless you have generous parents to make sure you want for nothing? if not the task is harder if your working to keep yourself a float and study chess? Best of luck

Probably 5 years.