However, depending on the context, you could consider these alternatives:
* For "young woman": "man", "boy", "gentleman", "fellow"
* For "girl": "boy", "lad", "young man"
* For a sense of youthfulness: "elder", "old man/woman", "senior citizen"
To give you the most accurate opposite, I would need to know more about how you're using the word "lass". Please provide me with some context!