Morgan Levine and Steve Horvath devised a method to calculate phenotypic age and other measures based on standard and low-cost markers found in a CBC/metabolic panel plus C-reactive protein.

References

An epigenetic biomarker of aging for lifespan and healthspan
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5940111/ 

An epigenetic biomarker of aging for lifespan and healthspan", Aging (Albany NY) 10(4) 573-591 (2018 Apr 18).
https://www.ncbi.nlm.nih.gov/pubmed/29676998

Journal: An epigenetic biomarker of aging for lifespan and healthspan", Aging (Albany NY) 10(4) 573-591 (2018 Apr 18).
https://www.aging-us.com/article/101414/text

Morgan Levine published the original PhenoAge calculator using 0.090165 as the denominator, but corrected it to 0.09165 in this paper (https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1002760)

Calculations here were derived from the outstanding spreadsheet by John Cramer that were adapted from the original paper.

Some values on the web page may be somewhat different than the spreadsheet.  Although the page displays numbers only to 2 or 3 decimal pages, the code behind it goes to a much greater number of decimal places than the spreadsheet.  

For me, what's most important are changes after an aging intervention therapy.

LinComb = linear combination of variables times weights that it the final input that generates the mortality scores and ages.
MortScore = Mortality Score (probability of death in the next ten years)
Ptypic Age = Phenotypic Age, i.e., your apparent biological age as implied by your blood variables. 
est. DNAm Age = apparent DNA methylation age
est. D MScore = revised estimate of probability of death in 10 years, based on the estimated DNAm age.
 

 

↑ or ↓ better (JA: I suspect up to a point)

Albumin: dissolved proteins in the plasma, including hormones and other signal molecules.
Creatinine: a waste product cleared by the kidneys, thus a high value suggests kidney malfunction; but it can be confounded by exercise, which raises creatinine.
Glucose: blood sugar rises with Type 2 diabetes and loss of insulin sensitivity.
C-reactive protein: this is a measure of systemic inflammation.
Lymphocyte %: the most common types of white blood cells.
Mean red cell volume (MCV): the average size of red blood cells
Red cell distribution width (RDW): standard deviation of the above
Alkaline phosphatase (ALP): this is elevated in liver disease, including cancers and hepatitis.
White blood cell count: total white blood cells of all types