CYAN'S GROWTH AND THE TIMEFRAME IN SPAWN
Spawn #3: (First appearance) She can say "mama," walk but immediately wants to be carried by Wanda and doesn't seem to say anything else.
Spawn #5: She sleeps in a crib and seems to only say "mama" and "don't" between babbling and laughing. Also, she can run a little bit but still relies on Wanda to carry her right away. It looks like her teeth have come in, but I'm sure not all of them.
Spawn #6: She loves being picked up and played "airplane" with; she just giggles and doesn't say any other words.
Right around their first birthday, a baby starts dropping single words with actual meaning ("mama," "don't"), enjoys gross motor play (like playing "airplane"), crawls or runs very clumsily while still preferring the safety of their mother's arms, and is right in the middle of teething (which is why she seems to have teeth, though she's likely still missing her molars).
Spawn #12: I'm sure she's depicted in diapers here. She can say "nope," "chess" (yes), "sha-shaw" (Shanna, her dog), "home" (while pointing at her house), "pooh" (her stuffed animal), and "dadt" (while running clumsily toward her dad). She still wants her parents to carry her at the slightest thing and holds Wanda's hand to walk. She also drinks from a baby bottle.
Spawn #13: Still sleeping in a crib. (I'm pretty sure everything from #13 to #15 happens during the same day/night, but I'd need to re-read it more thoroughly; I'm just skimming and relying on memory here).
The emergence of jargon or "imitative babbling" ("sha-shaw" for Shanna, "dadt," "chess") combined with pointing gestures is textbook behavior for a 13-month-old. She's still in diapers, drinks from a baby bottle, sleeps in a crib, and holds Wanda's hand to steady her walk.
Spawn #24: We only see her sleeping in Wanda's arms.
Spawn #27: Still relies on Wanda to carry her, has a tantrum, and you can tell she's missing teeth. She eats in a high chair, falls asleep in her dad's arms, and wears one of those onesies designed to accommodate diapers.
Spawn #28: Still falling asleep in her dad's arms, and now that I notice, her hair is always kept quite short... (I think only one day has passed since the previous issue).
Spawn #29: Still stuck on "Momma" and "Dadt," but she knows how to jump onto her parents' bed.
Using a high chair to eat, having tantrums out of frustration (classic when they want to communicate but can't yet), wearing full-body footie pajamas over diapers, and falling asleep easily in someone's arms confirms she still a baby. The short hair is totally normal; many babies' hair grows very slowly during their first year and a half. The milestone of "jumping onto the bed" shows she has more leg strength and better body control for climbing, which is typical for 14–15 months.
Spawn #35: She mimics her mother playing on the phone and starts using crayons to color. She can also say "gan-ma" (grandma) and likes being tickled: "again! me do again".
Spawn #36 She knows how to form short sentences (questions): "why are you crying?", "Did he make you sad?", "Does my mom-ma know you?". Her hair is still short, but they might just be keeping it trimmed.
Her ability to color with crayons and ask short questions ("Why are you crying?") pushes closer to 17–18 months in the real world, but physically, she's still a short-haired baby. That is why I would place it in the 16–17 month range.
Spawn #39: It's Christmas. We only see Cyan in a photo being held.
Spawn #42: Looks like she still wants to be carried: "Mama. Up. Peez! Up."
Cyan is still asking to be carried ("Mama. Up. Peez!"), showing that walking long distances still tires her out. Her phrase "Mama. Up. Peez! Up." places her closer to the Spawn #35 than the Spawn #36, so I wouldn't want to let too much time pass; I would keep her in the 16–17 month range yet.
Spawn #45: She might be eating some kind of treat at the doctor's office. Wanda visits the doctor due to the dizzy spells from issue #42 (not much time can have passed).
Spawn #46: She wants her dad to pick her up. She finally has pigtails! Little buns. In the previous issue, her hair seemed longer (so maybe it's just natural hair growth with little styling).
Spawn #50: In her high chair playing with her food and spreading it around: "Oh-oh. Mess. Mommy, mess," she playfully apologizes. Still wants to be carried and asks her dad in the hospital to come home: "Daddy! Go home!"
Her hair has finally grown long enough to be tied into two little buns/pigtails (very common at 18 months). Playing with food ("Oh-oh. Mess.") and apologizing is classic behavior for exploring and testing her parents' reactions at this age.
Spawn #50 (2nd story): I think there's a time jump of a week or months in the plot. Cyan now has a little braid and is picking her nose.
Spawn #51: She looks a bit bigger now, can't really tell if she's wearing a diaper. Her hair is long enough for a ponytail. They ask her about Shanna and she answers: "Going stinky poo-poo." She seems to have a fixation on pacifiers, even though she's never been seen with one before. Since there's something mystical going on in the plot, maybe McFarlane is reflecting that age where kids refuse to let go of them.
It is starting with the second story in Spawn #50 that we see a time jump regarding Cyan. Her fixation on the pacifier (used here as a plot device) leads me to believe that, from this point on, her age likely falls within the 18-24-month range.
This is a good breakdown for now. Just keep in mind that in Spawn #54, Cogliostro refers to an event from Spawn #7 as happening "months ago," so years cannot have passed, otherwise he would have said "years ago." Furthermore, in Spawn #58, Andy and Eddie (the abused kids from Spawn #29) arrive in New York, and it's implied that not much more than two months have passed since that event involving Spawn.