As noted in the forum post you reference, the error for BIRTHYR in ACS data is either 0 or 1. I will follow up with the IPUMS USA team to request that the documentation on the comparability tab clearly reflect this. For some research applications this single year error may be problematic. Unfortunately, there is no way to more accurately estimate year of birth given the detail available in public use ACS data. You could certainly use different data that allow you to get at birth year more accurately assuming you don’t require variables or geography that are specific to ACS data.