Are these tax liens or judgments?
It is true that most derogatory items age of your report after 7-1/2 years. Even chapter 7 bankruptcies stop appearing on your report 10 years from date of first delinquency (see this for more information).
Satisfied (paid) tax lies remain on your report for seven tears from the date released (paid), while unsatisfied tax liens can remain on your credit report indefinitely.
Judgments age of your report after 7-1/2 years or until the SOL, which ever is longer. However, most states allow un-settled/unpaid judgments to be renewed. Do not count on that judgment just going away at the 7 year point
If these items aren’t tax liens or judgments then a simple dispute process with the credit bureaus will take them off. See credit-report-disputes.html for a good guide.
As for your score – I’m not sure that it’s low because of these items. Credit scores are 90% based on the last two years activity, and by the time old derogatory items age off your report, they have very little affect on your score.
Your score may be low because of more recent negative information, high credit utilization or simply because of lack of positive information.
Hope this helps.