I agree with the others, it's more ability based than age IMO. My girl has been doing "toddler" things since 9ish months. That's when she started walking, and her vocabulary exploded. She's 11 days away from her first birthday and speaks in (short) sentences. "It's good" "I love you" "I did it!" ect.
I also think it has to do with their "look" My cousin has a baby a month younger than Lila, but he's bald as can be so that makes him look more babyish than my girl who has always had a head full of hair. He's also a big ol chunk of baby! He's got the most adorable baby rolls, and my girl has always been bitti. He outweighs her by 3 or 4lb, and so does my nephew is 3mo younger he's also a big baby.
I think toddler is when you start seeing them as little people vs babies.
Baby Lila ( End of May)
Toddler Lila (end of June)
Those pictures were taken a month apart (to the day!) but she's babyish (imo) in the first one and definitely toddler in the second.
Current Lila (end of August)
Maybe my girl is a bad example.. she's always kinda been a little person!