Is there a minimum age for trade-holders?

Technically, there's no age minimum. It probably wouldn't be realistic for a month-old pup to earn a trade but as long as you complete the relevant threads, your character's eligible. When they're young, they're more of 'apprentices' than anything, e.g., a five-month-old pup could possibly earn the hunter trade but since wolves don't typically hunt large game until they're older, they'd probably earn it via catching mice and that sort of thing. Just as long as it's (semi) realistic and reasonable. Does that make sense? :)