The Gemara states (Avodah Zara 57a) : "Rav Kahana and R. Asi said to Rav: You yourself said that a one day old Nochri makes Yayin Nesech (if he touches wine, even though he has no intention)! Rav: I meant only that one may not drink it, but one may benefit from it." Shmuel disagrees and holds that "only adults make Yayin Nesech, but children do not."
Rambam (Hilchos Ma'achalos Asuros 11:5): If a Nochri touched wine without intent, or a Nochri child touched wine, one may not drink the wine, but one may benefit from it.
Shulchan Aruch (YD 124:1): A Nochri child who does not mention idolatry or Meshamshim, if he touched wine he forbids only drinking it, but not benefit from it.
i. Beis Yosef (DH Lo): 'One day old' refers to a child who does not know well idolatry or Meshamshim. The Rashba says that knowing them well is when he remembers and mentions dolatry or Meshamshim with his mouth. A Tosefta says that an adult is one who vows (to idolatry) and mentions it and Meshamshim. R. Chananel, Bahag and the Ri rule like Shmuel. R. Tam rules like Rav; it seems that the Rosh agrees. The Ran says that the Ramban agrees, and so rule the Rashba and Rambam. We follow the Rosh and Rambam.
ii. Prishah (1): The Tosefta means that he mentions idolatry with his mouth, and through this we know that he remembers it in his thoughts. Therefore, even if he did not say anything when he touched, since we hear him regularly mention idolatry, presumably he was thinking about it at the time he touched.
iii. Taz (1): I say that 'with their mouths' refers to remembering and mentioning. I.e. he says it with intent. This is why the Shulchan Aruch says only 'mentions'.
"With their mouths" is a quote from the Tur, who defines an adult for these purposes as someone who is familiar with idolatrous practices and frequently says idolatrous things even if they haven't been directly observed practicing idolatry. "Mentions" refers to the fact that the Shulchan Aruch doesn't refer to someone who merely thinks about idolatry.