Neither. The statute of limitations runs from the date of last activity which is usually either the date of last payment or the date of the last charge. You can read more at my blog http://www.californaidebtblog.com which also discusses this and provides the Statute of LImitations for many states.
The California statutes of limitations start to run when you have defaulted on an obligation, not when you have incurred that obligation. Whether or not the debt has become "uncollectible" is, to me, not the relevant question. If you defaulted on a loan payment and since have not cured the default, the date of the default starts the running of the staute of limitations.