PBs lose meaning as you age because you slow down. Age Grading removes this age-factor and is a good substitute for PBs. I just added a page to my training site on Age Grading.
29 years ago, at age 46, I set a PB of 19:45 in a 5K. To reach that PB again, I'd have to cut off 11-12 minutes, an impossible achievement for a man my age. However, Age Grading tells me that to reach the same body-condition that I had 29 years ago, I would only have to cut off 4-5 minutes, a feasible achievement for me if I train hard and don't have any more health problems. The numbers for this speculation are given in my page on Age Grading.