Is the 了 in 你几岁了？ a change-of-state 了? There's a simple question we ask kids:

你几岁了？
How old are you?

It's possible to ask it without the 了, i.e., 你几岁？.  This makes me think this 了 is a change-of-state 了.  It may just be a kind of &quot;decorative&quot; 了: something like 啦.
If this 了 is a change-of-state 了, then (to my understanding) this question can only be asked on the kid's birthday, as per this question.
Question: Is the 了 in 你几岁了？ a change-of-state 了?
Yes, you can understand 了 in 你几岁了？ as a change-of-state 了, and it's a more natural way to ask for age than 你几岁？
Explanation:  你几岁了？ can be understood as aging is a process and this is asking which 'extend' has this process been for you, in this context, how old. And 你几岁？ would be simply asking for a property.
A bit similar to:
你几岁了？ -&gt; How old have you become, over all these years?
你几岁？ -&gt; How many ages are you, at this moment?