Children were certainly sent to sea at the age of 11 or even younger, as we see with Nicholas.
I think you're absolutely right that Nicholas would have been seen as an adult-in-the-making; a great deal must have depended on social background - the poorer a family was, the more likely its children would have been sent to work at an early age, as a point of necessity. This applied to the poor in Victorian times as well (children working in mines/factories etc.) Nicholas was well-off enough to be educated, but those around him would have seen nothing unusual in a boy of his age having to make his own way in the world.
The idea of a child-centred society (for all, and not just the well-off) is something I see very much as a late 20th Century concept, a luxury enabled by post-war improvements in lifestyle and was perhaps only just developing as AF was writing the Players novel - so possibly an easier shift in perspective for her than for the 21st Century reader.
Re: A nit-pick/voice breaking
I think you're absolutely right that Nicholas would have been seen as an adult-in-the-making; a great deal must have depended on social background - the poorer a family was, the more likely its children would have been sent to work at an early age, as a point of necessity. This applied to the poor in Victorian times as well (children working in mines/factories etc.) Nicholas was well-off enough to be educated, but those around him would have seen nothing unusual in a boy of his age having to make his own way in the world.
The idea of a child-centred society (for all, and not just the well-off) is something I see very much as a late 20th Century concept, a luxury enabled by post-war improvements in lifestyle and was perhaps only just developing as AF was writing the Players novel - so possibly an easier shift in perspective for her than for the 21st Century reader.