What is your advice for young people struggling with sexual daydreams?
The Prophet, 
, says:
God has pardoned for my community what comes to their mind, so long as they do not act or pronounce words to that effect. (Sunan Abi Dawud 2209, various forms of this hadith are also present al-Bukhari, Muslim and others)
Sexual fantasies are a natural consequence of having high levels of sex hormones. They are not sinful in themselves, but they can lead to sinful behavior. If they become a nuisance then fasting or reducing calorie intake is said to help. You can also search for “how to lower sex drive” and many articles will come up on dealing with an overactive sex drive.
And if they are causing you to do sinful things then this answer may help: What is the best way to avoid habitual sins?
Best wishes.