It's all about the maturity of the person, not their age. People also have different adjustments, views and expectations to sex depending on their upbringing, experience and childhood etc
It's been my experience, that girls of around 18, wouldn't be that uptight or squeamish about it , but it also depends on the nature of the conversation. Guys (like myself
) often get a bit too farcical or lewd about sex as well (especially around teenage years, when they're just learning about sex and going a million miles an hour) which is just unnecessary.