Is obedience to parents obligatory when they refuse their son's marriage to a girl he desires, on the pretext that she is older than him, this coming after the son and the girl repented from the fornication that occurred between them?

Adultery is one of the most heinous sins, but Allah accepts repentance and replaces evil deeds with good ones. Repentance involves abandoning the sin, regretting it, and resolving not to return to it, while concealing one's past. The marriage of a repentant adulteress is valid. If the girl has repented, there is no impediment to marrying her. You are not obliged to obey your parents if their refusal is solely due to her past as an adulteress, especially if you would be harmed by leaving her. However, if their refusal is for valid reasons concerning her religiosity or character, then you must obey them. If you would not be harmed by leaving her, then it is preferable to seek someone else whom your parents approve of.