The "old enough" question could have relevance in relation to the courage of back-office generals who don't have to fight the front-line battles and face the bullets of the enemy. There are people like Michael Novak who were pro birth control until they turned 45 and then decided they were going to stand with the Holy Father from then on. But if someone has generously responded to God's offer to send them blessings, then their age is irrelevant, if for no other reason than that the parenting days are never over. Parents of 8 or more children rarely if ever have to experience the "empty nest" syndrome.
The NFP teacher training question is relevant because it means that you've been indoctrinated. The moral issues do not require training and experience teaching NFP. I don't believe that Pope Paul VI had either. But once someone has gone through one of those programs, then they will have had the party line drilled into them, and it will be much more difficult for them to be objective.