Brain is not fully mature until 30s and 40s

My biology professor told me this at like 13/14, AKA a decade ago.

He said something along the lines of; "When we enter puberty the connections in our prefrontal cortex grow exponentially before seemingly randomly it collapses, but it takes into our thirties before our brain reaches an equilibrium and nowhere is really growing or diminishing... until you hit my age [late 50's] and then everything is going down hill."