For the purpose of maturity, the point in time by which to begin measuring the age of a language is the first point at which a stable release appears for general availability. Go is 9 years old, and Rust is 6.
If we're going by design inception dates, Go goes back to 2007 or earlier.
Not necessarily the troll's motivation, but: there's also a lot of "Rust envy"[1] going around from some people (mostly disgruntled C++ devs who feel threatened by it) who'll baselesly say anything and misrepresent something to prove their point. How does it never cross their mind that they're on the wrong side of things if they have to grasp for lies to prove their points?
[1]: I came up with the term just now, but I'm not sure "envy" is the right word; maybe "petty spite" is a better term.