My toddler followed the typical "when she can do (blank) she'll mellow out" pattern. She was a very fussy, demanding infant who always wanted to be held/worn. She had silent reflux, and diagnosis and treatment made a difference in her behavior - but she was still grumpy and exhausting. She really, really wanted to be mobile. As soon as she was pulling herself up and cruising around furniture around 6 months, she was more content... and walking improved her temperament exponentially. At 18 months, I would say she isn't exactly mellow - she's very active, very communicative, and still likes to be involved in everything - but as far as fussiness goes, there's hardly any. When she's not confined, she's a very happy kid.
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My first DD, weirdly enough, followed an opposite pattern. She was a very laid-back baby - great eater, great sleeper, happy to sit and watch the world go by. Then around age 3 she developed serious separation anxiety, night terrors, and a very demanding, emotional temperament. At 13 she is still a pretty demanding, intense kid.
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So - who knows. They're all unique.