What does '来' means when it is used to describe age?
Base on the two different answers from user6065 and steveLangsford, my conclusion is, "the dictionary definition of 来 really should be a '概数' (approximate number) synonym to '上下' or '左右'. Which mean 二十來岁 = 二十岁左右 (approximately twenty)

But in practice, most people see 来 as a synonym of 几 and eventually became accepted fact that "二十来岁" means "二十几岁 (twenty something)"