[1] https://stallman.org/archives/2012-nov-feb.html#04_January_2...
Also note that it is a difference between an empirical claim and a moral claim. While empirical claim that some action almost always leads to negative outcome to others could be used as an argument for that action being immoral, it is still perfectly valid to attack the supporting empirical claim, while not attacking the moral claim.
There are Stallman's statements defending non-coercive sex with children, but they are older:
https://stallman.org/archives/2003-may-aug.html#28%20June%20...
At least there is this one from 14th September, also on his site:
https://stallman.org/archives/2019-jul-oct.html#14_September...
" Many years ago I posted that I could not see anything wrong about sex between an adult and a child, if the child accepted it.
Through personal conversations in recent years, I've learned to understand how sex with a child can harm per psychologically. This changed my mind about the matter: I think adults should not do that. I am grateful for the conversations that enabled me to understand why."
Citation needed. He expressed the same opinion in 2013 that he did in 2006 and 2003:
https://stallman.org/archives/2012-nov-feb.html#04_January_2...
https://stallman.org/archives/2006-may-aug.html#05%20June%20...
https://stallman.org/archives/2003-may-aug.html#28%20June%20...
But even if he hadn't changed his opinion there would be nothing wrong with that imo.
The stuff I did find was only tangentially related and not indicative of any changes in position, eg
https://stallman.org/archives/2016-jul-oct.html#16_September...
https://stallman.org/archives/2016-jul-oct.html#12_September...
https://stallman.org/archives/2016-jul-oct.html#15_July_2016...