Biomarkers of aging to allow short-term measures of how biologically (as opposed to chronologically) old an individual might be are very important in the future development of treatments for aging. The only present way to determine effects on longevity is to wait and see, which is enormously, painfully expensive in time and money in mice, and out of the question in humans. Short-cutting that with a test that you run before and after a few weeks or months of a putative rejuvenation therapy would greatly speed the process of experimentation and assessment, and time taken in development is of great importance when (a) we're all still aging to death, and (b) there are many promising early-stage approaches to repair and reverse the underlying causes of age-related degeneration.
This is why there is great interest in, for example, DNA methylation patterns that seem to correlate very well with age. I'm not aware of anything else that is presently as promising as a biomarker of aging.
http://onlinelibrary.wiley.com/enhanced/doi/10.1111/acel.123...