Why come in the last episode of season 4 will said he was not the same 14 year old he was when he came to stay with the banks, wouden't he have been around 18 in season 1 of the FP ???

https://www.youtube.com/watch?v=n3eJkf4erbI
https://www.youtube.com/watch?v=n3eJkf4erbI
https://www.youtube.com/watch?v=n3eJkf4erbI

In season one, Will looked like he was only 15, so it would make sense if he was playing the role of a 14 year old.