Currently the randomize/open slot to add villagers for a building will just randomly add any laborer into that worksite. Personally I think there should be an option to both keep the random fill any open slots, but also being able to select one of the villagers in a list to give them specific professions. This way you can choose and decide whether or not that 16 year old is suited to work as a guard, or that 60 year old should work as a hunter.
On that note, age should be a factor in how well a person works. Elderly can’t carry much, healthy adults at age 20s to 40s walk and perform tasks a certain rate faster based on their specific age etc.
Adolescents won’t be able to work as well as ordinary adults would, for realism and all that.