They may be referring to the old hippie slogan "Don't trust anybody over 30", meaning they're no longer young, and have started into middle-age.
Being a very active, rock-n-roll-playing 65-yo myself, I looked at 40 as the beginning of middle age, and as far as I'm concerned, my "middle age" will continue until I damn well decide it's over. Maybe around 80, God willing and the creek don't rise.
Oh I remember the slogan...those who used it thought thirty was decrepit