I think misguided is the right word, re putting this in the Sun. Presumably the mother thought it might be a good way of normalising / promoting longer term breastfeeding, but unfortunately I don't think the majority of the audience is really in the right place to receive that message! It is also unfortunate that her child is being splashed across the Sun in this way.
I breastfed my first child until her fifth birthday (where we mutually agreed she would stop), and my second to about four years and three months (and he self-weaned). I would not want this fact to be shared with the Sun, or any other national newspaper for that matter. Post toddlerdom it was a v private matter and down to my and my children's choices.
It is certainly unusual to feed to this age, but it is culturally unusual (in the UK at least) rather than biologically unusual. I think sharing that some women do breastfeed beyond toddlerhood, and it isn't some kind of weird or shameful issue, is absolutely fine and important. Unfortunately I don't think the Sun is the right medium to convey that message! And I don't really like the idea of putting pictures of your breastfeeding child out in the public domain fullstop.