First of all thank you for the reply.
H/W ver.:A1
Firmware Version: EU_1.03
Country: Italy
Youtube Safety Mode is explained here: https://support.google.com/youtube/answer/174084?hl=en. It is a mode to protect children from watching videos not appropriate for them.
It is based on cookies. According to this source (http://webapps.stackexchange.com/questions/42106/how-to-enable-safety-mode-in-youtube-via-url-or-cookie), it seems that in order to force it, one should append "&f2=8000000" to the "PREF" cookie.