I had hepatitis a over 12 years ago as an teenager, why are you not allowed to donate blood, when the virus is completyly out of your blood in a year?

Likely other reason.: At this point in time, to my knowledge, hep a infection that has resolved is not an indication not to donate blood. The immune system is effective and there is vaccination. However, you may have had another infection/issue, like hepatitis c which has no vaccine, and can be transmitted. Also, if you went to europe, you might have been exposed to "mad cow" disease, and cannot donate again.

Hepatitis and blood: The red cross disqualifies anyone who has had any hepatitis (a, b, c, d or e) or jaundice after the age of 11. Exceptions include non-viral causes of hepatitis or jaundice such as gilbert's disease. Presumably the reason is to cast a wide net over all cases of viral hepatitis to ensure not missing a case of non-a hepatitis.