Rubio would have won it against Hillary. Trump probably loses it, but not as badly as Romney, largely because the current 18-24 crowd is somewhat more conservative than the previous one.
You got any proof of that? Anecdotes and feelings dont count. Also, Trump is far less popular than Romney, so...
https://youtu.be/s3sFeOlZ9zM?t=15m27s
CPAC attendees are a tiny, self-selecting sample. I see little reason why Rubio or any other Republican would be able to win young millennials, who among other things are even less white and less religious than older millennials. There's also zero evidence, in polling or otherwise, that Rubio would have any special appeal to non-Cuban Latinos.