Parenting plan by youngest or oldest
<p>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. </p>

<p>Courts consider the ages of all children obviously - what else would make sense?</p>

<p>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).</p>

<p>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 (<a href="https://www.gesetze-im-internet.de/bgb/__1627.html" rel="nofollow noreferrer">Bürgerliches Gesetzbuch, §1627ff</a>):</p>

<blockquote>
  <p>§ 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.</p>
  
  <p>§ 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. [...]</p>
</blockquote>

<p>English translation (by me):</p>

<blockquote>
  <p>§ 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.</p>
  
  <p>§ 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. [...]</p>
</blockquote>

<p>In short, the legal answer is: There is no specific rule, the court must decide each case on its merits.</p>

family-law divorce parental-rights