Transgender people can be godparents at Roman Catholic baptisms, witnesses at religious weddings and receive baptism themselves, the Vatican's doctrinal office said on Wednesday.
There's also a few denominations of Christianity (albeit protestant ones) who only baptise teens and adults, as they believe baptism has to be a conscious choice rather than something done to an infant in order for it to be valid.
Yeah, almost mentioned them but they wouldn't be covered by the Pope's decisions, but you're right - and I assume in some catholic communities they might have similar practices even though that's a minority case.
There's a separate ceremony called Confirmation to let the person choose to believe when they're older, in both Catholicism and Protestantism. Usually at the age of eight to twelve, so it's not exactly a choice then either, but they could theoretically choose not to.