First, In French, “jeunes garcons” is ambiguous and not the same as “young boys”. Young boys refers to boys before 13. In French, it means teenager before 18, or younger, it depends.
I’d say that “little boys” is from 5 to 10, “young boys” from 10 to 18, and young man “jeune homme”, from 18 to 24 or so.

Second, Frederic Mitterand is a person that “builds up” on words. He exagerates, all the time, about anything. Any event in his mouth is transformed as spectacular, innovative, incredible. He describes reality in an exagerated way.

So … Has he exagerated while describing young men into young boys? I don’t know. He paid for sex, for sure. If he really had sex with young boys, he should be judged.
About Polanski, if he raped a 13 year’s old girl, I think he should be judged, and that’s it.

Let’s not forget that the far right raised this question by reading a chapter of Mitterand’s book on TV, and changed the word “young boys” into “little boys” to make it more scandalous. The far right always intends to drop the confidence into moderate politics.
So I’m careful and waiting for proofs before deciding if someone is guilty.