I have never seen any signage at Tescos to say there's an age-limit on using their P&T spaces, nor have I ever shopped at one using the permit system - which is, incidentally, probably the most sensible way to deal with this.
After all, a sign that simply says 'Parent and Child' is open to interpretation - I have wondered, idly, if I could use them if my mum was in the car with me (not seriously, honest).
From what the lady concerned has said in her responses to the article, I gather that it's not the first time that she's used these spaces with her child, and she's never either been told that he's too old or seen any signage to tell her that he's too old.
Given that, and given the lack of information from Tescos to users of the carpark about a change in policy, I suspect that she's got a very good chance of fighting this fine.
That said, I would love to see Tescos and other supermarkets fining those who misuse P&t and disabled spaces.