Duolingo simulates the deterioration of your vocabulary over time by unstrengthening previously learned skills over time, as shown below.
As you can see, the skills that are not golden are not fully strengthened.. I always tend to strengthen all of my existing skills before moving on to newer ones, but I'm never sure whether to start with the most recent skill (in this case, Negatives, or the earliest learned but unstrengthened skill (in this case Clothing).
Does it make a difference which one I choose to review first? I always tend to review my earlier skills first, as I can more easily remember the later learned skills. Note that I only ask this in general, with Duolingo as an example.