At a minimum, keeping track of grades and credits can start in 9th grade. Grades 9-12 are considered high school, and those are the only grades and credits that colleges want to see. 

I will often suggest that parents start keeping homeschool records in middle school, however.  This is important for two reasons.  First, middle school is "parent training time" for keeping homeschool records.  You will be able to figure out a good system prior to high school, when it really counts.  The second reason is that as homeschoolers, you won't always know precisely when your student is entering high school!  They may begin doing high school level work in one or more subjects early.  It is always better to have those records already done than going back and recreating them (a practice I refer to as "forensic record keeping.")

Most (but not all) colleges will accept homeschool grades and credits, given by a parent, typed on their home computer using nothing fancier than a Word document. In my experience, most (but not all) colleges will use your credits as real credits *if* you have test scores to back them up. If you don't have some evidence that your grades and credits are true, then colleges tend to look only at your test scores, and look at the transcript as if it's just a list of classes, rather than real numbers they can use for scholarships or placements.  Grades and credits can be determined as you go along, or at the end of each school year.  Often your grading standards will evolve as you homeschool for awhile so it is always a good idea to review all the grades and credits that you have given at the end of each school year to make sure they accurately reflect your homeschool.