People saying 40 are being deliberately disingenuous because they are jealous and there is a culture on Mumsnet of women wanting to tear each other down. Basically. So if anyone asks something like this, some women think they deserve an obnoxious response.
Nonsense. Why would people be jealous of an internet stranger.
I guessed 40 and gave reasons why (crows feet present, for example), but I also made sure to compliment her good skin.