The Gemara explains that according to Rebbe "Yedeas Beis Rabbo" is considered knowing. This means that since when a person is young he is taught that one can contract tum'ah in various ways, therefore when he is older and came in contact with tum'ah, he is considered having been aware that he was tam'ei. This is true even if at the time he touched the tum'ah it didn't occur to him that he was tam'ei. This needs explanation. The lesson though, is clear. Things we are taught when we are young really never leave us, and as much as we are unaware now, we can always connect back to our previous knowledge.