[{"Age":"27 yo","Age_Standardized":27},{"Age":"81.0","Age_Standardized":81},{"Age":"15 male","Age_Standardized":15},{"Age":"25 years old","Age_Standardized":25},{"Age":"unknown","Age_Standardized":"None"}]

db.collection.aggregate([{$set:{Age_Standardized:{$regexFind:{input:"$Age",regex:"[0-9]+"}}}},{$set:{Age_Standardized:{"$ifNull":["$Age_Standardized.match","None"]}}}])