I've noticed in a few spec scripts I've gotten recently that some writers aren't giving enough thought to casting when writing their scripts. Now before you say, "Wait a minute! Aren't you always telling us to not offer casting suggestions?" let me explain what I mean.
Not long ago, I read a script that had no fewer than four major roles for characters under 12 - and two of those roles were under six years of age. It was a very weighty drama with a lot of the emotion depending on the ability of some very young actors. I have to admit, years of enduring bad writing has made me very wary of spec scripts with young protagonists just on principle. It's hard to write young kids without being cloying, and too often, it feels like green writers fall into the trap of concocting "cute" things for their supporting kids to do.
Generally, if I read a spec with one child actor in it, it doesn't usually trip my alert. I figure that there must be at least one child who can be found that can fit the bill. Also, if the film has a lot of adults in major roles, there's the expectation that their acting might compensate for any weaknesses with the younger actors. Plus, every now and then filmmakers get lucky and stumble on the next Jonathan Lipnicki (you know, from Jerry Maguire) or Dakota Fanning. Odds are you can find at least one talented kid. Last season, I was very impressed with a young actress named Ariel Winter, who appeared in a multi-episode ER arc as the daughter of a woman in need of a heart transplant. (She's currently on Modern Family.)
But the rub was that this script was about as heavy a drama as they come, and I'd wager that a good 70%-80% of it was based on the interactions of these kids. With two of them being between the ages of 5-7, that had me concerned that finding the right child actors could be a hassle.
Oh and all of these kids were siblings - so in addition to all of that, you had to believe they were related too. (I could be wrong, but I'm pretty sure there were also lines remarking on just how alike the kids looked - which probably would have been cut if the script got any further.)
I'm not saying that casting this film would be impossible, but the success of the project would be resting on some very small shoulders. My advice might have been to cut the five year-old and age the seven year-old up to being nine or ten. Given the restraints of the premise, it would have required some rethinking, but it would have kept me from thinking, "How are we going to find a five year old who can say and emote this convincingly?"
I'm less worried about the other end of the spectrum. If you can't find a 90 year-old actor, you just age-up a 70 year-old one. However, I do seem to recall reading once that M. Night Shyamalan wrote himself into a corner with Lady in the Water, when he insisted on finding a particular ethnicity for a role, despite being told that there were few choices in that demographic. (I think he was looking for an overweight half-Asian woman, but I'm unable to locate a copy of the book The Man Who Heard Voices: Or, How M. Night Shyamalan Risked His Career on a Fairy Tale, which covers this in detail.)
So while this isn't a major issue, and casting people sometimes work near miracles, give it some thought the next time you write a five year-old half-Asian/half-Samoan set of fraternal twins who are crucial to the plot.