Paradox said..
Depends on how you look at it....Decades are blocks of 10 years and are counted different to individual years. He apparently started in the 60's and we have just started another decade this year, even though we are only on month 2 of it, it is still the 7th decade he has been playing, even though he might have only actually played for say 55+ years. (60's, 70's, 80's, 90's, 00's, 10's, 20's)
Provided he started performing in the 60's then he has been indeed been rocking for 7 decades,
His profile says he started a band in high school, he was born in 1948, so he may well have almost scraped in 8 decades if he started a band in the last few years of primary school instead....
I see your point, but I think it's very, very charitable and still ... wrong

The actual wording: "Cooper [...] [has] been delivering [shock rock] shows [...] for seven decades now. Cooper [...] is 72 years old."
7 decades *is* 7 x 10 = 70 years. He's 72, so he'd have had to start when he was ... 2.
You *could* say he was entering his seventh decade of performing (61 *actual* years), but that would still mean he started doing the Alice Cooper thing since he was 11.
Saying "his career has spanned seven decades" I would accept. I don't mind a little hyperbole for dramatic effect.
But that goes to my point here: the level of ability to accurately use the English language in a group of people that are supposed to be well versed in word smithing is disheartening. And probably reflects their level of comprehension too.
It's the whole thing of saying your 20 because you're in your twentieth year, even though your birthday is 11 months away. You still aren't getting in the pub.
(Is it still 20?

)