The oldest child may have learned responsibility before learning how to decline it.

Competence learned early often arrives without its counterpart. Somebody who was needed at eleven becomes very good at taking weight and never gets to practise the sentence for putting it down, because there was no occasion when refusing was an option. What is missing is not backbone. It is a skill that was never taught, in a household where it would not have helped.

If that is you, practise the small refusal somewhere a no costs almost nothing. If you love somebody like this, ask rather than assume, and let either answer stand without consequence.