A reckless technique that vaguely "reduces lifespan" when 1) required to save his brother 2) required to save himself from dying from the poison is not the same as literally giving up your entire childhood to join in a fight that you are not needed in, is not the same thing.
Also Luffy doesn't mind being dangerously reckless himself, but he wouldn't want others to do so, especially 8 year old kids.