I read from this link (http://www.ada.org/1887.aspx)
It says to:
Replace toothbrushes at least every 3–4 months. The bristles become frayed and worn with use and cleaning effectiveness will decrease. Toothbrushes will wear out more rapidly depending on factors unique to each patient. Check brushes often for this type of wear and replace them more frequently if needed. Children’s toothbrushes often need replacing more frequently than adult brushes.
I had a question was that is this replacement require if the bristles were not frayed and worn even after 6 months of usage? Or are there scientific evidence showing that it is a must to replace the tooth brush every 3 to 4 months?