There are several actors and actresses who won the National Film Awards at a young age, they are:
Monisha Unni:
She won the National Film Award for Best Actress at a very young age of around 15 to16 years in 1986 for the Malayalam movie Nakhakshathangal.
Unfortunately she passed away at a very young age of 21 years in 1992 in a car accident.
Shoba:
She had won the National Film Award for Best Actress at the age of just 17 years for the Tamil film Pasi.
She also passed away at a very young age of around 17 years in 1980, she took her life by herself.
Riddhi Sen:
He won National Film Award for Best Actor in 2018 at a young age of around 20 years for the Bengali film Nagarkirtan.
In this film he did the role of Parimal, a trans woman from rural Bengal. Riddhi Sen is a Bengali actor who also worked in some Hindi films.
Smita Patil:
She won her first National Film Award among her 2 National Awards at the age of 22 years in 1977 for the Hindi film Bhumika.
Shabana Azmi:
She has won 5 National Film Awards for Best Actress. For the first time she won the award at the age of of 24 years in 1974 for the Hindi film Ankur.
Suhasini:
She won the National Film Award for Best Actress at the age of 24 years in 1985 for the Tamil film Sindhu Bhairavi.
Tabu:
She won her first National Film Award for Best Actress at the age of 25 years in 1996 for the Hindi film Maachis.
Mithun Chakraborty:
He received National Film Award as a Best Actor at the age of 26 years for his debut film Mrigayaa in which he did the role of Ghinua, which was released in 1976.
Priyanka Chopra:
She won the National Film Award for Best Actress at the age of 26 years in 2008 for Bollywood film Fashion.
Keerthy Suresh:
She won the National Film Award for Best Actress at the age of 26 years in 2018 for the Telugu film Mahanati.
Rekha:
She won the National Film Award for Best Actress at the age of 27 years in 1981 for the film Umrao Jaan.
Dhanush :
He won the National Film Award for Best Actor at the age of 27 years for the film Aadukalam which was released in 2010.
Kamal Haasan:
He won the National Film Award for Best Actors for 3 times. For the first time he won at the age of 28 years for the Tamil film Moondram Pirai which was released in 1982.
Alia Bhatt:
She received National Film Award for Best Actress at the age of 28 years in 2021 for the film Gangubhai Kathiawadi.
Naseeruddin Shah:
He won his first National Film Award among 3 National Film Awards at the age of 29 years for the Hindi film Sparsh which was released in 1979 in which he did the role of Anirudh Parmar.
Ajay Devgn:
He won the National Film Award for the first time at the age of 29 years for the film Zakham which was released in 1998.
Rajkumar Rao:
He won the National Film Award for Best Actor at the age of 29 years in the year 2013 for the film Shahid.
Vicky Kaushal:
He won the National Film Award for Best Actor at the age of 30 years for the film Uri: The Surgical Strike which was released in 2018.
The above are the actors and actresses who won the National Film Award at the age of before 30 years.
Actress Monisha Unni is the youngest actress who received at 15 or 16 years and Riddhi Sen is youngest actor to receive National Film Award at the age of 29 years so far.
As a child artist, the youngest to win the National Film Award is Treesha Thosar, she won the award at the age of just 4 years in 2023 for the Marathi film Naal 2.
Here one can remember that the veteran actor Kamal Haasan as a child artist won the National Film Award at a very young age of 4 or 5 years for the Tamil film Kalathur Kannamma which was released in 1960.
Even there may be many other child artists who proved their talents in acting and won National Awards when they were around 10 years.