Reddit's ToS require all users to be 13 or older. There is no other requirement to be a moderator, and moderation is independently managed by each subreddit. Reddit tends to be very hands-off with interfering with who moderates a subreddit, for both better and worse.
On the internet nobody knows you're a dog.
... until it matters, and issues like "Minors can't enter into contracts" result in someone getting their account unceremoniously ping-ponged around because they have fewer rights than an adult.
They don’t ask your age. (Except probably “are you over 13” at account creation).