You don't provide any details of where you are "going back" to, but given the general arrogance of Americans to assume that the USA is the only country there is I will assume that is where you originate (please correct me if I am wrong).
Given that assumtion I have never ever been questioned regarding my age when purchasing duty free alcohol. I have however been asked about ID many times in the US (and I am 29) so this tells me that duty free shops are more relaxed than the average bar/restaurant in the US. Regarding going through the US customs I have never been stopped there either (or in any other customs for that matter), so I assume that your chances of going through there with alcohol are better than your chances of purchasing alcohol at a US bar.