When does disobeying parents become grave matter?Whenever they ask you to do something that is for your own good and you do not do it, you sin. And, since "Honor your father and mother" is the first of the Commandments that concern love of neighbor, this sin is also quite grave. From the Catechism:
2217 As long as a child lives at home with his parents, the child should obey his parents in all that they ask of him when it is for his good or that of the family. "Children, obey your parents in everything, for this pleases the Lord" (Col 3:20; Cf. Eph 6:1). Children should also obey the reasonable directions of their teachers and all to whom their parents have entrusted them. But if a child is convinced in conscience that it would be morally wrong to obey a particular order, he must not do so.Any sin against one of the Ten Commandments is considered grave matter (cf. CCC, 1858) and this is one of them.
As they grow up, children should continue to respect their parents. They should anticipate their wishes, willingly seek their advice, and accept their just admonitions. Obedience toward parents ceases with the emancipation of the children; not so respect, which is always owed to them. This respect has its roots in the fear of God, one of the gifts of the Holy Spirit.