In modification agreements, do the courts go by the youngest or oldest childs age when deciding parenting plans. The age desparity is by 4 years difference being 2/6.
Courts consider the ages of all children obviously - what else would make sense?
If the age disparity is sufficiently great, the court may order or suggest different parenting plans for the children - for example, I know a (separated) family where the older sibling moved from mother to father during puberty (though this case did not involve a court).
Ultimately, many factors play a role when finding a parenting plan, and age of the siblings is only one of them (though an important factor), so there is no general answer. In particular, the law in most countries has very few concrete rules for handling this, usually the law simply says that the decision must be "in the best interest of the child". For example, according to German law (Bürgerliches Gesetzbuch, §1627ff):
§ 1627 Die Eltern haben die elterliche Sorge in eigener Verantwortung und in gegenseitigem Einvernehmen zum Wohl des Kindes auszuüben. Bei Meinungsverschiedenheiten müssen sie versuchen, sich zu einigen.
§ 1628 Können sich die Eltern in einer einzelnen Angelegenheit oder in einer bestimmten Art von Angelegenheiten der elterlichen Sorge, deren Regelung für das Kind von erheblicher Bedeutung ist, nicht einigen, so kann das Familiengericht auf Antrag eines Elternteils die Entscheidung einem Elternteil übertragen. [...]
English translation (by me):
§ 1627 The parents must exercise parental care under their own responsibility and with mutual agreement in the best interest of the child. In case of disagreement they must try to find a solution.
§ 1628 If the parents cannot find an agreement for a specific issue or type of issue, whose regulation is of substantial importance for the child, then the family court can, on demand of one parent, assign responsibility to one parent. [...]
In short, the legal answer is: There is no specific rule, the court must decide each case on its merits.
It is always best to draft a parenting plan that caters to all children and can be used for the next 3 - 5 years without any party having problems with it.
Joint parenting is about being mature and responsible and working cooperatively with the other parent to ensure not one person is at a disadvantage. Because if parents focus on fighting each other than the kids will suffer even if the fighting is about who has the best interest of the child.
Courts will look at the youngest child to see how you guys plan to cater the needs of the youngest child since they require more attention and cooperation from parents.
Are you drafting the parenting agreement yourself or using a professional? If you are doing it yourself then follow the above general advice on the first sentence.
Be flexible, and make sure you put the Children's Best Interest Forward it will automatically make the raising children in a joint custody easier.