He's in decline because he's 31?
Ibrahimovic right now is playing the best he's ever played and he's 33, Zidane was sublime at 30-33 years of age as well, and many more players. Van Persie was injured a lot when he was younger, so he still should have plenty left in the tank, you act like his game relies heavily on pace when it clearly doesn't. Also the bit about him declining from January 2013, where exactly did you get this precise date from lol?
He definitely needs to start scoring some goals now, I'm talking braces and maybe some hat-ricks, until his goal record goes completely sour then I refuse to believe he's in a steady decline, and it most definitely isn't because of his age which isn't even that old by the way.