The "and eleven and a half months" is not considered in your formula. You can do direct calculations on dates, so consider this as an option:

=IF(ISBLANK([Date of Birth]@row),"",ROUNDDOWN((TODAY()-[Date of Birth]@row)/365,0))

Dividing by 365 is done because the math between the two dates gives you the number of days. Rounddown, as implied by the name, rounds down. The ISBLANK and IF are the determination of whether there's something in Date of Birth.

Remember that sheets don't always automatically calculate. The sheet must be opened, or some kind of change must be implemented on the sheet (e.g. form submissions, update requests, or specific automations) to cause recalculations. So if you are using this sheet to do things like populate a dashboard, you'll want to do something to force recalculation. Good luck!