I've found guys I was about to give up on suddenly start rocketing around the end of their sophomore beginning of their junior year as the incremental increases in their work ethic from playing time finally hit the sweet spot, which seems to be at least in the upper half of the twenties and more.
At least lower than that almost always seem much slower than higher than that. So at least to me, the cut off point seems to be around twenty five, if you plan on them being anything other than a bench player for their career.