If there is only a single age field, you record the ages of 0 to 11 months as zero. (Usually you can find this instruction in the field helps. Field helps are opened by clicking on the purple question mark that appears when you click on the field.) If the only indication is the word "infant" then you do not index an age as infant can mean anyone under the age of majority, a minor in other words.
Some projects might have instructions to round up if there are day, month, and year fields. It kind of depends on what project you are working on.