I believe part of the reason why it's typically done on younger people is letting the implant grow into their bodies with time, but it took Ein very little time to be fielded again which I guess can be attributed to superior Gjallarhorn medical technology and probably a rush job to field a skilled pilot in a good machine. But yeah Orlov could've just been kept under medical supervision for a few years while his body adapted to the implant. Given that he's a commander he'd likely have a pretty good standard of medical care.