How to Say the Age of a Baby or Child ("Rule of 12")

How should we say how old a child is? A 2-day old infant is is 2 days old. But a four weeks old baby is not a 30 days old. Nor is a 14 month toddler a 57 week old.  Like everything we measure the age of a child is best stated in relation to the magnitude. Nobody measures his or her own weight in tons or mg.

I will propose that we summarise the standard practice according to some descriptive rules.

1.  0-14 days : express age in days.
2. 14 days - 13 weeks (3 months) : express age in weeks
3. 3 months - 12 months: express age in months and half months (e.g "8 and a half months") 
4. 12 months - 24 months : express age in months after the year (e.g. "a year and three months")
5. 2 years - 4 years: in years with half years (e.g. "2 years and a half")

These rules seem ok, and they seem to follow standard practice. But I was looking for a simple, parsimonious rule. After looking at different options, I propose: Rule 1: Do not use any number higher than 12. Notice that this rule would not match the cutoffs of the originally proposed rules, but it would be a good approximation. What about the half months and half years? We can add Rule 2: Use half days, weeks, months, years, as granular units. This means that 6 months would hit Rule 1 as 12 half months. How would the descriptive rules above change?

1. 0-6 days: age in half days.
2. 6-12 days: age in full days.
3. 12 days - 6 weeks: age in half weeks (e.g. one can say "3 weeks and a half")
4. 7 weeks - 12 weeks: age in full weeks.
5. 13 weeks - 6 months: age in half months (e.g. "4 and a half months")
6. 6 months - 12 months: age in full months. 
7. 12 months - 6 years: age in half years. 
8. older than 6: age in full years.

I am quite satisfied with my parsimonious Rules 1 and 2. I still think it is ok to add the months to children between 1 and 2 years old, but that would make the rules too complex.