The youngest I have seen is where a man had his "brother" who was listed in the records as a "schoolboy" for one of the sponsors. He had just turned age 10. I've seen a number of records were the sponsor's identity is certain from a stated relationship in the baptismal record, that the sponsor was in the 14-16 age range (usually a sibling, or nephew/niece of one of the parents). These are all Canton Bern, in the time period 1750-early 1800s. Rick Saunders