According to the Child Status Protection Act a person can apply for permanent residence as a child if they were beneficiary of a visa petition approved or pending after August of 2002.
My question is that if an I-130 petition was filed with in 1996 and given a priority date of January 1998, does that mean the application was approved in January of 1998 or when the visa number became finally avaialbe in Febuary of 2009?
I'm not sure your terminology is correct. I think you mean that it was approved in January 1998 after being filed back in 1996, in which case your priority date is the date the petition was filed. If the sponsor was a LPR, then you subtract that time between '96 and '98 (less than 2 years) from the age of the beneficiary when the visa became available (Feb 09). This is why I aged out of CSPA benefits.
If the sponsor later became a USC, the beneficiary's CSPA age is the date of naturalization. This is because there is no waiting time for visas for a single child under 21 being petitioned by a USC parent. If the sponsor was a USC when the I-130 was filed in '96, then the visa was immediately available when the petition was approved in '98.
Note: All this may not even matter since this all happened before 2002. I'm not too familiar with the rules for old petitions.