Rust was started in 2006, it had already undergone 9 years of development by 2015. in fact, it reached stability in 2015. According to Mojo's changelog, it was

Rust was started in 2006, it had already undergone 9 years of development by 2015. in fact, it reached stability in 2015. According to Mojo's changelog, it was started in September 2022, which makes it less than 2 years old in active development... but even then, it'll look a lot like its final form by year's end.