I thought it might be because a lot of developmental milestones occur when children are around toddler age, and it's seen as easier to compare what stage kids are at milestone wise using dates? e.g., 1 & 1/2 is simple enough, but anything less than about 1 & 1/4 might get confusing.
I may be wrong, though.
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Blackbird singing in the dead of night, take these broken wings and learn to fly...