Shach (Yoreh Deah 246:8): … Chagiga (15b) states that the reason why R’ Meir learned Torah from a heretic — despite the requirement that a teacher be like an angel — is because that prohibition applies only to those who will be influenced by the teacher. R’ Meir and others who are capable of withstanding the influence are in fact permitted to learn from a heretic… The question is why didn’t the Rambam note this distinction that the gemora makes between adult and child—between those mature enough not to be influenced and those who might be influenced? It is possible that the Rambam agrees with Tosfos (Chagiga 15b) who notes that in Moed Koton (17a) R’ Yehuda excommunicated a certain scholar because he had a bad reputation. Tosfos explains that the reason that no differentiation was made by R’ Yehuda is that he felt all the students would be influenced. Thus, we see that even in Talmudic times the older students were considered as susceptible to bad influence and had to be protected. So surely, in our days, everyone is considered as susceptible to bad influence and thus the distinction of the gemora is not relevant for actual Halacha. An alternative explanation is that most poskim do not accept this distinction because it was only held by R’ Meir and not the majority of our Sages.