A menudo encuentro esta frase y entiendo vagamente su sentido, pero ¿qué significa exactamente y cuál es la mejor traducción al inglés? Gracias.
"Sentar" means, among other things, to settle down. The word "cabeza" just reinforces (and clarifies) this idea, by specifying what exactly about you is staying for good and for a reason. So it is not just any part of your physical body that it is staying living in a fixed place, but your brain maturing enough to decide what to do, and stick to it. In other words, it means to become mature enough to know what to do in life.