What are the ranges wherein a baby's age can be referred to in terms of weeks, months and years respectively? Is there a set threshold that people conventionally use, or does there exist a "threshold period", an overlap between ranges?

Whilst not quite an overlap but an example of regular exceptions, are the dates where a multiple of 4 weeks or 12 months correspond to an integer amount of months or years respectively. For example, over the course of 2 weeks one could start referring to a baby as "3 weeks old", a week later "1 month old", and a final week later as "5 weeks old".

Save these exceptions, does there exist a hard threshold between using weeks and months; and months and years?

  • 2
    There is no hard rule. A few hours is comparable to a day, a few days to a week, a few weeks to a month, a few months to a year, a few years to a decade and few decades to a century. I rather like Jarod Kintz's “Days turn into weeks, which turn into months, which turn into years, which turn into decades, which turn into cemeteries".
    – Anton
    Dec 3, 2020 at 12:08
  • I've been thinking about this recently as I have a young dog! Dog owners frequently ask the age of each other's pets. There is a lot to this question. It would probably take an essay to answer fully. There are all sorts of approximations such as "Just over 8 weeks", "Nearly a year", "She'll be a year old next Tuesday", etc, etc. A general rule is whether the calculation is difficult to make mentally so, "She is 57 weeks 5 days" would obviously be wrong. Except for medical reasons, no-one wants that precision. I'll give it some thought but I may never get around to answering fully! Dec 3, 2020 at 13:14
  • I don't know much about young children, but a bit of googling confirms my impression that people often talk about toddlers' ages in months, even after two years - presumably because they're developing so quickly at that age. 'At X months, your toddler will...' Dec 3, 2020 at 13:23


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