my condolences

Technically no. Giftias are full-grown the instant they are made/manufactured (they don't age in other words, as they are reset back to full capability once they are retrieved). Isla in particular seems to be an adult Giftia of petite variety (keep in mind Giftias are meant to mimic real people, so much so that they're treated and viewed as real people. Many people who are full adults frequently appear physically similar to Isla's body type)

Not only that, but Isla in the series is treated like an adult. Meaning it is safe to assume, at least mentally, she is one.