I would wholeheartedly agree with that. I myself am 31, and cannot compare my energy levels to that of a 16 year old. I am in the Marines and have mandatory training I must attend, some of which is trying, some not so much. I try to do CF WODs Mon/Tues and Thurs/Fri. I also train Krav Maga or BJJ 2-3 times a week at night.
I can hardly keep this schedule, its hard to admit even to myself.
I want to delete that line, that's how strong the denial is. But being older and wiser now I am trying to train smarter not harder. I drop WODs from time to time. I try to roll at least 2 times a week, preferring BJJ over Krav Maga, but sometimes I only do fight training once a week.
Bottom line is you have to let your body rest.
Train smart and remember quality over quantity.