I have $20K in student loans across 8 accounts at 3 lenders dating back from 2004-2009 here in the USA. The average interest rate is around 6%. I can likely get a fixed rate around 3.5% if I refinance, clearly an improvement. I'd love to get these rates down so I can feel like investing my disposable income is more certainly better than paying down the loans. But I'm concerned about the effect on my credit.
My credit score is currently Excellent, partially because of these loans: the average age of my credit is 10 years, and my oldest account is 16 years. Refinancing will almost certainly reduce my credit, first because of a hard credit pull (or three), but possibly more because of changes to the age of my credit. By how much should I expect my credit to be reduced, and for how long? Is there some back-of-envelope calculation for this sort of thing?