MailChimp lists, triggers and automation

I currently collect birthday data, I want people to receive relevant mailings to the age of their child. So I need MailChimp to translate a birth date (18/01/2017) to their age (1-year-old), then filter that recipient into a particular position (1-2-year-olds) in a huge sequence of automated mailings.

I already have an annual automated mailing (1 month/ 1 week before their birthday) set up.

So I need to find a way to translate a date to an age. Then identify how a user would move from one 'List' to another.

I use tag process for the annual automated mailing mentioned above. This could work, however, the trigger appears to be limited to 999 days after the original date. I need the email trigger to be able to send more than that. For example, a parent receives tailored guidance about their child when they are over 1000 days old.

Also thinking I could segment my audience, but I need it to reflect today's date, rather than static dates. For example, today this user is 2, however, next week they are 3 and therefore will need to be automatically moved.

I've considered a Zapier/Google Sheets interaction. Currently, Zapier is inserting a new row, which annoyingly avoids the formula on sheet 2.

Help, please.