Conventions in referring to a baby's age

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?