One question that came up during the table-reading of Draft 1.0 of The Physics of Joy was if the character of Joy - who time travels between present day and twenty-three years in the future - should be an 8-year-old girl in the present, or should she be younger.

The feedback from the read, in which Joy was written as 8 in the present, was that her dialogue and actions were more consistent with those of a 5-year-old than with those of a girl of 8.

I agreed, and in the current drafts since that first table read, I've reduced her age down for a few reasons:

From a pure dialogue point of view, 8-year-olds are not radically different than an adult character; the conversations between them doesn't have strong cues to the audience that this adult actor before them is now playing as a small child. Also, for the actors and reader, I found there to be less to distinguish Joy on the written page. The types of conversations you engage in with an 8-year-old tend to be not too different than a polite or playful conversation with an adult, especially with girls who tend to mature faster than boys.
The 8/9-year-old girl in my family has been working so hard at being cool, and tends to be more reserved than effusive. She doesn't jump up and twirl any more. She does play with dolls, but I don't think she'd be crushed the way a 5-year-old would be if one of them broke.
The stakes feel lower when Joy is 8-years-old. For example, there is a scene where she walks home alone as dusk. While it may still be outside the norm, it is not uncommon for 8-year-olds to have liberal boundaries within their neighborhood and/or community. To have a 5-year-old do the same, even if it's only a couple of blocks, is alarming.
And last, frankly, as I noted that while I do have an 8/9-year-old girl in my family, I have a 5-year-old daughter around me day and night. As such, I kept finding I was writing a 5-year-old, whether I wanted to or not. This one is actually a lame excuse - I also don't know any Irish barkeeps or millionaire power couples - but it's an honest, self-reflection on a bit of why the character got to the state that she is in.

Playing Joy younger brings with it some challenges as well, though:

There is an important event that is happening in Joy's life during and surrounding the show. A 5-year-old probably won't hold on to these strong memories - even ones as big as this event merits - with the intensity that an 8/9-year-old would, and that colors who Joy is in the future.
Of greater concern to me, I lost having Joy's father, Evan, being a child dad. If Joy is 8-years-old, it's easy to make Evan be at the beginning of his twenties when Joy is born. This sets up a situation of children raising children. With Joy being 5-years-old, Evan necessarily becomes written as being in his mid-twenties when she's born - still young, but his brain is fully developed at this point. It means that - though human and capable of all the stupid mistakes any parent makes - it's less likely, and less forgivable by the audience, if he's an emotionally formed adult when raising her. The Prefrontal Cortex is the last part of the brain to develop, and in kids this happens in their mid-twenties. And this is the part that manages impulse control and judgment. Until it's developed, the Amygdala handles the judgment calls that will later be handled here; and the Amygdala is purely emotion - it's why asking a teen a trivial thing, like please make your bed, can turn into an emotional eruption. So while having Joy younger heightens the stakes for some of her own things, it risks lessening the stakes for some of the things that happen with Evan.
Last, as my friend Hannah pointed out, it's a challenging casting issue. It's likely easier to find actors who can convincingly pull of being both 8/9 and 28. Once 5-years-old is introduced, it starts to bring up ideas of double casting and actors passing the baton of the role during the time-traveling moments.

As I've been rereading and revisiting the decision to make Joy younger, I'm growing more confident that having Joy in the present being 5-years-old is the right thing to do. That said, I'm looking forward to the next table read to understand better if it's working as I think it is.