You can use Modified Java Script step to calculate age of each user.
Add Modified Java Script step in your workflow.
While reading, take datatype of DOB as date.
To calculate age, lets first take currentDate by using inbuilt Date() function of Java script.
Create new variable Age and call DateDiff() function. Pass required parameters.
First parameter would be DOB. Second parameter would be currentDate. In third parameter we need to mention either day or month or year in which we want to calculate the difference. We would mention “y” for year.It will give you age in no of years.