How long of a period of malnutrition is too long where catch-up growth wont be complete in a growing kid?

I would need to know which kind of malnutrition you are talking about. Vitamin, calories, fat, cHO or what. Get back with us with more details. By kid do you mean a young goat?

The growth centers of a male are usually closed at your listed age of 18, so more height is unlikely. A single x-ray of your wrist/palm can identify the status of your growth centers. If closed you are done & no more height is expected whatever your nutritional status. If open, a balanced diet would support more growth.