1. How old a prior still counts?
Ten years. The "priorability" of old DUI's (wet reckless and regular DUI convictions both count) used to be 5 years under former vehicle code section 23102. In 1981, penalties were increased and the current vehicle code section CVC 23152 was born. In 1986 the period was extended to 7 years, and in 2005 the current 10-year rule was implemented. The period of risk is 10 years from date of arrest (courts use the date of arrest, not date of conviction, when determining if priors are within 10 years and should be charged now, but courts use the date of conviction when determining if a violation of probation should be charged now).
Sometimes a prior incident counts for DMV suspension purposes but not in court, such as under-21 priors with low alcohol levels, or a prior was dismissed in court but the license was still suspended after the prior, or if the District Attorney "misses" a prior which the DMV computers separately identify and use in a new DUI incident.
DUI's can go in different directions in different cases with very different results. Click on Anatomy Of A DUI on this site and see the many different directions a Ukiah case can take, some of which result in priors, and some of which don't for certain purposes. Consult with a Mendocino County DUI lawyer about your specific case.