I'm sorry, but that's simply not accurate. There are absolutely circumstances under which previous convictions/bad acts are admissible, including juvenile records.
(d) Juvenile Adjudications. Evidence of a juvenile adjudication is admissible under this rule only if:
(1) it is offered in a criminal case;
(2) the adjudication was of a witness other than the defendant;
(3) an adults conviction for that offense would be admissible to attack the adults credibility; and
(4) admitting the evidence is necessary to fairly determine guilt or innocence.
http://www.law.cornell.edu/rules/fre/rule_609