Ah, the difference word placement can make… As the sentence is written, the babysitter is panicked, possibly as their usual state of being. Move panicked so it describes the call rather than the babysitter, and the meaning of the sentence is more precise/accurate.
I’m gonna use a real-life bio-science example to explain why the last sentence in this example is factually wrong: There’s a disease called progeria. It makes children who have it appear to be old. They’re not old, though; they’re still children, and many of them aren’t even going to live to see actual adulthood because they’re “aging” so rapidly. They haven’t lived through several decades of life even though they have many physical traits in common with senior citizens. A nine-year-old with progeria is still a nine-year-old child. Thus, this fictional baby who has “aged twelve years” in three hours (somehow without dying of starvation: where did the energy and nutrients for growth come from?*) is not a twelve-year-old and has not actually experienced twelve years of time.
(“But why does it even matter to change it, you can tell what the writer means?” *sigh* See the tags on this post? One of ’em is science. Believe it or not, careless use of words in fiction can lead to misunderstanding of facts, and the country where I live has serious problems lately with getting its citizens to grasp — or even accept as real — scientific fact, logic, and all that jazz. I prefer not to encourage anything that makes this worse.)
Adding looks like, seems, etc., will fix the problem with the example sentence; someone or something can appear to have aged even though the time has not actually passed.
Don’t use numerals in fiction prose. Replace 3 with three and 12 with twelve.
I strongly dislike referring to a baby as it. Babies are as much people as their parents are, and it is for inanimate objects and potted plants, not for people. If you don’t want to use a gendered pronoun, use they.
I think the second and third sentences would sound better if combined.
Add a comma after three hours.
You receive a panicked call from your babysitter. Something spilled on the baby, and in the last three hours, she seems to have aged twelve years.
(* Three works of fiction come to mind as I’m finishing this post. The first is Singularity, a novel by William Sleator, the guy who wrote Interstellar Pig and House of Stairs. The second is the short story “Lieserl,” by Stephen Baxter. The third is the short film “Jack-Jack Attack,” mostly because of the panicked calls from the babysitter: “Mrs. Parr, it’s me. Jack-Jack is fine, but weird things are happening…”)