Most infant guidance is based on a baby's age in weeks or months. From my very limited experience of parenting (7 months), it seems to me that it would make more sense to base any information and guidance on the weight and developmental milestones of the baby.
For instance, shouldn't guidance for sleep and food requirements for a 25 percentile 7 month old that crawls and is very active be different to that for a larger 7 month old which isn't yet very physically active?