I can't find dates of birth for Shirley and Mick, but there's 14 years between Linda Henry and Danny Dyer so I'd imagine it's similar for the characters they play (although I seem to remember hearing that Mick Carter is a couple of years older than Danny Dyer).
So yes, it is possible that Shirley is Mick's mum but I hope it's not the case, especially after the Kat and Zoe saga.