Short Answer
"you might state that you overheard a few sexist remarks and that this bothers you" - from the accepted answer by Frank Hopkins
Do not say this.
If you really do say something, talk about your feelings and how you took offense from an off-hand comment. They're your feelings. No one can argue against them. But do not label that behavior.
Only label the issue if you bring it to HR or if this becomes a legal issue. But otherwise, if you're just looking for a friendly apology from the person in question, don't use that label with him.
Longer Answer
Calling the remark "sexist" adds a degree of shaming that mayis likely to antagonize the person in question.
If the first and most prevalent impression about the presenter is his or her appearance, chances getting lowered he or she might be associated as a potential hiring candidate in the first place
You've reframed this issue as something more than it really is.
This woman isn't interviewing for a job at your company, nor does she even work for your company.
As a junior, giving a professional talk is a chance to prove...
And no, even if she was young, that makes her at the very least a senior engineer, not a junior one.
After all, not everyone gets invited to speak at professional conferences. And with the topic of machine learning, it isn't that uncommon to have young people be senior subject-matter experts on that topic.
...that has nothing to do with her professional competence." – Geoffrey Brent
"Remember the short bald guy. His head was so shiny!" or "He was so hot!" or "His voice was monotone." or "He was so creepy!"
My point being, as a public speaker at a conference, it would be nice if everybody remembered your presentation and discussed its content, but it doesn't always work that way.
With that said, I do agree that saying that "she was so hot" was inappropriate, and saying it like that in an open office space environment where six of you work does make it worse than trying to say the same thing privately.