The gemara states that a child who is lame and will never heal is certainly exempt from chinuch since he will never have a mitzvah even as a gadol, the question is only for a child who will eventually heal, is there a mitzvah so long as he is lame? The gemara concludes that a child who is lame one does not have to be mechanech since a gadol in that state is exempt. Rashi seems to contradict himself bec. Rashi explains that a lame who "will heal" means he will heal prior to becoming a gadol, which implies that if he will heal when he is 15 that is considered "will never heal". So, why does Rashi explain that "will never heal" means even as a gadol he won't heal?
Rashi seems to understand that if he will heal when he is 12 then there is no rationale to be michanech him now 1. in his present state he is exempt even as a gadol, 2. there will be time to be michanech him in this mitzvah after he heals. But if he will only heal at 15, then although the first reason doesn't apply, we must still be michanech him now since he will eventually heal while he is a gadol and not know how to do the mitzvah. Therefore, Rashi understands that even acc. to the conclusion of the gemara we only exempt a child who will never heal or will heal when before he becomes a gadol, but if he will heal after he becomes a gadol we must be michanech him now (even in a state of chiger).