Look, railing against Gotham for its absolute refusal to portray any of its characters in a consistent way is a fool's errand. But executive producer Ken Woodruff's explanation of why the show is turning Poison Ivy from a kid to a voluptuous, seductive 20-something — through what I believe is the magical power of creepiness — takes the cake. It takes 40 cakes. And that's terrible. Image: Gotham, Fox
In a Hollywood Reporter interview discussing the season premiere of Gotham, Woodruff explains the reasoning behind the decision:
We made the change for two reasons: The character Ivy in the comics, one of her greatest powers is the power of seduction. Everyone was much more comfortable with that with an older actress as opposed to a teenager. We want to explore that classic, canonical power of Ivy. And we didn't just make her older with that attack. When she's changed and transformed, there's a real character change as well. She'll still have some of the same traits, but she'll be much darker, more manipulative than the Ivy we've seen so far. There's a more evil quality to her as well. It's more than just physical.
I will give him credit for being pretty honest about why they aged her up: They wanted to have the sexy lady on the show. I will also give the show credit for understanding that a teenage femme fatale makes for uncomfortable TV. (Of course, the writers didn't think that through when they added Silver St Cloud to this mess of a show last season.)
On the other hand, isn't she still a child? Like, just because the actor isn't a teenager anymore doesn't mean the character doesn't still have the mind of a child. She hasn't lived extra years just because some creep used his superpower to age her up.
Plus, she isn't going to have the power of seduction just because her body is suddenly older. That's not, like, an innate thing women get when they go through puberty. If they wanted to have an Ivy that acts like an adult Ivy, maybe they should have thrown in the towel and just gone with time travel.
Same goes for being darker and more manipulative. Unless the magic that ages her up also changes her whole personality. Which would at least be an explanation, unlike every other massive character swerve on this show.
Let's also go into the idea that seduction is one of Ivy's "classic, canonical powers". It's not entirely wrong, but her signature power is manipulating plant life. That's what makes her Poison Ivy and not just the usual random crazy that infects all of Gotham. But sure, ignore the thing that literally gives the character her name and focus on her sex appeal.