Serious analysis and the result was as follows:
At the age of 20,
- A.R.Rehman : was in Illaiyaraja's troupe as a keyboard artiste, already a few rungs up the success ladder.
- Sivamani : finished 9 yrs of his drumming career already!
- J.K.Rowling : (I'm not quite sure if she was at 20 or 25) gets the idea of the story of a young boy attending a school of wizardry.
- Saina Nehwal : well, the hat-trick of titles-Indian Open, Singapore Open and Indonesia Open.
- Sania Mirza : wins the silver in Women's singles, gold in mixed doubles AND a Padmashri award!
- Leo Tolstoy : started writing... :|
- Thomas Mueller(the cute guy i mentioned,playing for Germany) : scores the fourth goal,beating England.
- W.Shakespeare : is married to a lady called Anne Hathaway ;) :P
- Robert Pattinson : famous for his role of Cedric Diggory in Harry Potter and The Goblet of Fire.
.
.
.
.
.
.
.
.
- Ranjitha Raghu : updates her blog three months after she's 20 :|
I'm sure there're many more people you can add to this list, but the fact remains that there's so much you can achieve before you're twenty, and a lot more AFTER 20 ;)
Cheers! :)
P.S.: Thanks to Venki, i did my homework :D