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?