You could solve this in XPath with the following constraint:
[Geboortedatum >= '[%BeginOfCurrentYearUTC%] - 18 * [%YearLength%]'] [Geboortedatum <= '[%EndOfCurrentYearUTC%] - 18 * [%YearLength%]']
See this page for a reference.
Seeing as you calculate the age with a microflow, can't you just also calculate (in the same microflow) if said person ''Turns 18 this year y/n'' and then just xpath on this boolean?
You need to have a datasource microflow for your datagrid. In that microflow you can determine what year it is with a token '[%CurrentDateTime%]'.
Substract 18 years from that and you know in which year the Customers would have to have their birthdate in. Knowing that you can do a retrieve with the following XPath: 'year-from-datetime($Attribute)'
For example. This year is 2015. So everyone born in 1997 is or will turn 18 this year
Your XPath would be something like: [year-from-datetime($Customer/DateOfBirth = 1997]
I did this from the top of my head, so please bear with me :-)
Obviously there are a lot of ways to get the results you want. This could be one of them..
You can use the xpath functions year-from-dateTime(). This will give you the year of the date value. After this you can substract by 18 and compare or the another XPATH operation.