add age reference to specimen schema

[lzehl]

cf. obophenotype/uberon#3488 (comment)
cf. also https://github.com/bids-standard/bids-specification/pull/1839/files

[apdavison]

This would be for SpecimenState, no? I would suggest creating an Age schema (to be embedded), containing a QuantitativeValue(Range) and the age reference (controlled term).