Your choice I think. My daughter turned four months (by the date) but 18 wks in the same week, so I keep track of both lol

We count by weeks. My background is in Education and Child Development, so we choose to keep track of his age by weeks, because all developmental milestones are listed by week. Our son is 18 weeks old. We consider him to be 4 1/2 months old. However, every parent is different and there is no right or wrong way to keep track of your child's age!

so yes he would be a month on the 15th. it's just like your b day.

I'd do the day because sometimes there are more days in the month than others.

Ive been going by the date. So the third if every month my baby is another month old. Some people do it the other way. Its really up to you which one you wanna use

Every four weeks