According to Jonathan Webber in Reading Sartre: On Phenomenology and Existentialism, the meaning is pederast:

[…] on 6 August 1942, the Vichy regime made homosexual relations with anyone under the age of 21 illegal. In light of this, a certain terminological drift observable in Being and Nothingness, as elsewhere in Sartre, is worth mentioning. Being and Nothingness uses the terms ‘homosexual’ and ‘pederast’ interchangeably, as does The Age of Reason, but in the latter work it is clear that Daniel – like the Autodidact in Nausea, like Bergère in ‘The Childhood of a Leader’ – is a pederast, in the sense that he is attracted only to adolescents and very young men.

^{Emphasis mine.}

According to the Dictionnaire historique de la langue française the word pédéraste has been used to mean homosexuel (and not a man who has sexual relationships with adolescent boys only) since the early 20th century.
But I remember the word being used by André Gide in the early 20th century to mean a man attracted by young boys.

In Being and Nothingness (1943) Sartre – the man and philosopher – uses homosexuel to talk about a homosexual and pédéraste between brackets to refer to exactly the same person, the brackets implying the word is derogatory in Sartre’s eyes. But it absolutely clear he does not use the word pédéraste to imply an exclusive preference for adolescent boys.

You will find an extract online here. The word pédéraste is used both times to reflect how other people (not Sartre) see the homosexual.

You must keep in mind that in the novel Sartre puts the words in the mouth of his characters, he writes the words but they are not his own words. In the extract I am pointing to Sartre talks about the shame felt by the homosexual. So you must ask yourself as you read: Who says what ? Is it Daniel? Is it another character? Is it the author?

I do not remember if in L’âge de raison Sartre uses the word pédéraste as Sartre, the author, but if he does, he probably did not try to stress Daniel liked younger men (even if he did). And we must also remember that Sartre fought for gay & lesbian rights (as we say nowadays) and I presume he would not use the words lightly.

The word pédéraste is hardly used nowadays in France. According to the Dictionnaire culturel en langue française it was superseded by homosexuel in the 1950-60s. Only the short form pédé is used nowadays as a very derogatory term and a severe insult.

Il est difficile d’affirmer quoi que ce soit dans des telles circonstances ; vu que Daniel n’est pas attitré pour émettre des jugements dans le domaine sémantique, que l’on se doit de concevoir sa déclaration comme celle d’un profane, on peut accepter celle-ci comme les mots de quelqu’un qui s’exprime « à peu près » et ne pas chercher à élucider.
Il faut comprendre tout de même que l’âge maximum des jeunes mâles victimes (ou « complices ») qui caractérise la pédérastie atteint 19 ou 20 ans (réf.), un âge auquel l’être humain masculin, s’il n’est pas encore en possession de sa pleine maturité est un homme adulte. Il faut aussi comprendre que si l’on ne fait jamais cas de considérations taxinomiques dans ce domaine on doit finalement s’en soucier et conclure qu’il est irréfutable que le pédéraste est un homosexuel (réf.). Alors, qu’importe vraiment la recherche de précision dans les paroles de Daniel ?

Enlish version of the above

It is difficult to assert anything in such circumstances; as Daniel is not entitled to pronounce truths in the domain of semantics, as we must understand his assertion as that of a layman, we can then accept it as the words of someone who is speaking in an approximate fashion and not try to make out the exact concept referred to.
It is necessary, nevertheless, to understand that the maximum age of the young male victims (or “partners”) that caracterises a sexual or rather pseudo sexual act as relevant of pederasty is as high as 19 or 20 years (réf.), age at which a male human being, if not yet endowed with his full maturity, is a fully grown adult. It must also be realised that if never anyone considers of any importance taxonomic matters in relation with these two appelations, one has finally to contend with that and conclude that without question a pederast is a homosexual (réf.). On the count of these facts, how does really matter a research for precision in Daniel’s words?

Note Only the French Wikipédia mentions age limits relative to certain cultures; the English Wikipédia states as a limit age that of becoming legally an adult (ref.).

I believe that your comment after your question is catching the main point, but not the whole picture. First of all, it is true that the generally accepted view of homosexuality in these times was to assimilate it to pederasty.

But there is a not-so-subtle addendum, which makes the situation far worse: the social view of homosexuality was so bad that it was a choice of the Society as a whole entity to label it as an infamy and to deliberately assimilate it to crime and exploitation of children. It was not an accidental choice, nor a random mislabelling, it was the social manifestation of disapproval of homosexuality. That awful assimilation was imbedded in the day-to-day language and contributed to the guilt and marginalization of homosexuals.

Nowadays in France, one of the most common words used to refer to homosexual is “pédé”.

I would refrain from using it though as it is most of the time used in a derogatory manner.

And although most people using the word ignores it, the word “pédé” does indeed come from the word “pédéraste” which has completely fallen out of favor.

While this does not constitute proof that the word “pédéraste” in Sarte’s “L’âge de raison” should be taken to mean “homosexual”, it is at least a hint.

That being said, I seem to remember that it is made pretty obvious in the book that Daniel is not a “pederast” but is a homosexual. By that, I mean that there is no passage where his potential desire for children is mentioned (I did read that book a long time ago).

As someone already pointed out, the word “pédéraste” has had the meaning of “homosexual” ever-since the beginning of the 20th century.

And this meaning is actually an extension of its original meaning. Even though it only encompasses a male to male relationship while homosexual also include a female to female relationship.