I suggest you first check with your client on what he/she is going to do with it and what kind of reports he is going to generate.
The implementation varies based on the reports, you can use classifications something like name:dob:location and create classification builder to build buckets ( 20 - 30 ), ( 30 - 40 ), etc. but this also be problem when it comes to next year.
(or) you can use separate eVar for this, what you need to do is here, you need to calculate age at front end and pass it to the registration event but the problem here is it will not updated every year. the important thing which you need to check is what he/she wants, current DOB or age at the time of registration?
I never tried for dates but, you can also try creating segments for age groups
