Hello Happier
I see your point. The issue is that the age check takes into account the actual age at the time of purchase and not the actual age at the time of ticket validity. The product page has been reworked recently, it seems that this use case was not considered by the developers. I reported it to SBB (POSP CS015494706); the issue appears on the website only.
I suggest you purchasing the Saver Day Pass using the SBB Mobile app. There, go to tab Shop & Services and select the Saver Day Pass from the Quick Access list.
Kind regards,
Roland