Update README.md

README.md

@@ -44,4 +44,12 @@ We discard all cells for a `cell+tissue` type if:
 For each donor, all cells were randomly assigned to exactly one split: train (70%), validation (15%), or test (15%).
 
 ### Mapping `development_stage` values to numeric ages
-Coming soon!
+
+Age information in CELLxGENE is derived from the `development_stage` field. 
+
+* Some values of `development stage` give a precise age in years.
+  * Example: `80 year-old human`. In this case, we assign a numerical value of `80`.
+* Other values of `development_stage` are broader.
+  * Example: `child_stage`. [It turns out](https://ontology.archive.data.humancellatlas.org/ontologies/hcao/terms?iri=http%3A%2F%2Fpurl.obolibrary.org%2Fobo%2FHsapDv_0000081) that this is synonymous with the age range of 2-12 years. In this case, we assign a numerical value of `7`, corresponding to the midpoint of the range. 
+
+This means that some of our numerical age values are more precise than others. This is reflected in the `age_precision` variable, which gives the maximum error in the assigned value of `age`. For instance, for `child_stage` we have a value of `5` for `age_precision`, since the assigned age (`7`) could be 5 years too low (i.e. age `12`) or too high (i.e. age `2`).