Adenoids are just a wad of lymphatic tissue at the back of the nose and form part of a lymphatic defence against infection. The back of the nose is small in a child, and hence the adenoids can cause blockage and subsequent problems. As one grows older, the adenoids remain the same size but the space at the back of the nose gets larger allowing more space for the adenoids which subsequently become less troublesome. So it is not so much that the adenoids shrink, but that the space(naso pharynx) gets larger. Biopsy is precautionary and perfectly routine treatment.