15-year old teenager from Finland, 35 years ago

He was 21.

As Torvalds wrote in his book Just for Fun, he eventually ended up writing an operating system kernel. On 25 August 1991, he (at age 21) announced this system in another posting to the comp.os.minix newsgroup

https://en.wikipedia.org/wiki/History_of_Linux

I don’t know if I would call Linux the most professional, but it’s definitely the biggest.