Back to the Law of Hywel Dda again, with some snippets about coming of age.
Young children up to seven years old were considered to have no legal capacity or judgment: "until [the child] is seven years old it is right for its father to swear and to pay on its behalf ... [because] the child has no judgement." "A daughter, after she is baptised, until she is seven years old, is not entitled to take an oath." The father is bound to make good any damage the child does, just as for animals. "From the end of its seventh year, it is for the child itself to swear for its acts, and for its father to pay."
Maturity for boys was fourteen. "From when the son is born until he is fourteen years old, it is right for him to be at his father's platter, with his father as lord over him. And no punishment of him is right save his father's." If his father should die before he is fourteen, the lord appoints a guardian for him. When the boy turns fourteen, however, "it is right for the father to take his son to the lord and commend him to him. And then it is right for him [i.e., the son] to do homage to the lord, and to be dependent on his lord's status; and it is for him himself to answer on his own behalf to every claim that is made against him ... his father is from then on no more entitled to strike him than a stranger."
Girls, it was recognized, mature earlier. "From when she is born until she is twelve years old it is right for her to be at her father's platter. From twelve years old ... she is ... of age to be given to a husband; and from then on, even if she does not take a husband she is entitled to control what is hers, and it is not right for her to be at her father's platter unless he himself wishes it."
I have a theory that because people were legally recognized as adults so much earlier, there was no teenage rebellion to speak of in medieval Wales.